Some car malfunctions are most apparent when starting off, when the damaged components are under increased strain. Knocking sound when moving off most often indicates critical wear of suspension or transmission elements, as well as trivial loose fasteners.
About what can be the source of extraneous sounds in the front and rear when starting off, what problems usually occur in specific models and whether it is possible to drive if there is a knock when starting abruptly, we will tell in this article.
Causes of a knock when starting a car
At the beginning of movement, the clutch, gearbox, transmission drives, working elements of the suspension, joints and supports are exposed to increased loads. As parts wear, they become excessively loose and come into contact with the mating parts.
Insufficient wear is the main reason why a knock is heard when starting off, but not all the time. In some cases, the symptom may also occur during sudden braking and stopping, when similar loads in the opposite direction occur.
Not detected in time, the cause of knocking at startup can develop into more serious problems and even pose a danger in the continuation of operation. Thus, failure of steering mechanism and brake system elements is fraught with sudden loss of control. Below we will consider in detail, why there is a knock when moving the car, all causes of its occurrence we will analyze in details, and also we will list options of their diagnostics.
Front end knocking when moving off
Front end knocking when moving off is most often caused by worn rubber-metal joints (engine or transmission mounts and silent blocks), steering components, and suspension mounts. Sometimes it may be caused by more exotic failure, such as severe wheel imbalance. If a number of parts fail or the fasteners become loose, the knock when moving away occurs at the front, rear, or on both sides simultaneously, depending on where the faulty unit is located.
Knocking Noise From The Front — Possible Causes Listed
The main causes of knocking in the front of the car when starting:
- Steering tips.
If your car has a knock on the steering wheel when starting off, the first thing you need to do is inspect the steering components. Broken ball joints of the tips, as well as loosened nuts of their attachment to the bipods, lead to the appearance of the impact in the steering mechanism at startup. When driving, the car may wiggle and a wobble occurs.
To find the backlash you can lift the wheel and swing it horizontally, observing the joint of the tip — it should sit firmly in the bipod hole and not move relative to the rod. The average cost for fixing such a problem is $250 to $300.
- Ball bearings.
Ball bearings, on which the moving part of suspension is attached at the bottom, in the presence of backlashes, emit a knock in suspension when moving off at the moment when the ball touches the outer casing of the pin. The noise will be especially noticeable when turning the steering wheel.
To diagnose it, you need to jack up the wheel and try to rock it in the vertical plane. Worn ball bearing will show itself with a knock, but the worn out hub bearing may also be the source of backlash. Its deterioration is usually accompanied by rattle when cranking the wheel and rumble in motion. Repair in the service will cost about $350.
- Arm Silent Blocks.
Rubber-metal joints in case of wear or damage acquire excessive freedom of movement. Their bushings can contact the metal of the body, thereby generating a knock at the front when moving off.
The wear on the silentblocks is checked by visual inspection (the rubber should be intact, the bushing should be in the center of the rubber) and by wiggling against the mating part with a mounting spatula. The knocking will sound like metal touching metal. It will cost about $500 to replace.
- Stretch rod silent blocks.
Macpherson front suspension stretch rod rubber mounts limit the longitudinal movement of the front suspension. When they are worn or broken — the wheel begins to drive forward before the body and at the moment of compression of the rubber band there is a shock.
How To Fix a Noisy Suspension
To diagnose, you need to examine the rubber elements («daisies» on the arms, «crabs» bushings, etc.) for damage, and check the tightening of their fasteners. Often too worn (or loosened) strut supports not only make a single knock when starting the car, but also are accompanied by the wheel rubbing against the wing liner when braking.
- Strut supports.
The upper supports that hold the spring and shock absorber lose elasticity due to wear. As a result, the bearings hit the outer part of the support in the «cup».
It is possible to estimate a condition of supports, visually inspect them and check the course by rocking up and down and sideways. Failure of prop supports and shock absorbers may be observed by driving on a rough road. Repair of such a problem may cost up to $750.
Sometimes the source of knocks can be stabilizer, shock absorbers and springs, but defects of these parts manifest themselves not only by extraneous sounds at startup. Stabilizer which has lost elasticity leads to roll in turns and swaying, its cushions squeak in motion, and struts knock on bad roads constantly.
- Engine mounts.
Engine pads, which have defects in the rubber damping part, sharply compress at the moment of startup. Because of this, the shock load is transmitted from the engine to the car body.
You can check the condition of the cushions by inspecting their rubber part and applying force to them with a mounting spatula.
- Engine protection.
The crankcase protection can touch the engine, gearbox, and other components, causing knocking.
To check it, you need to rock the engine by hand, listen to the sounds and see if there is no contact with the shield. If the deformations are small and you can not touch it manually, then you need to inspect the shape of the shield, its fasteners, check the fasteners tightening. To inspect it you can dismantle the shield: in the places of contact of motor or box you will see abrasions.
How to Check a Wheel Bearing (Sound, play in the wheel, ABS light)
- Hub bearings.
A loose hub bearing nut often causes a knock at the front of the car when sharply moving off. Rear axle knocking is also possible.
To check the hub bearing, jack up the wheel vertically and horizontally. Replacement costs $200 to $400.
- Exhaust system hangers.
When the exhaust line loosens, its elements can touch other parts when you move off, generating a knocking noise.
Inspect the exhaust system hangers, or lift and wiggle the exhaust line by hand to check.
- Loose fasteners.
Loose bolts and nuts of wheels, arms, struts, stretch bars, stabilizer, and other suspension components, show up with a knock. To identify it is enough to lift the car and check the fasteners tightening.
What can knock when moving off in the box or transmission?
The knocking when starting off and when shifting gears often appears due to transmission malfunctions. On front-wheel drive cars, the knocking sounds come from the front, but on rear-wheel drive and four-wheel drive cars, the knocking may come from the rear when you move off.
- Clutch disc. Clutch knock on startup is caused by wear of the damping springs, which transmit the force from spline hub to the outer friction part of the disc. Normally they should be smoothly compressed smoothing out the shocks, but if the springs are loose or cracked the torque is transmitted sharply and a dull knocking occurs when moving off and shifting gears. As the disk is hidden by the gearbox bell jar, the gearbox has to be removed to diagnose it.
- Transmission support. The gearbox at the moment of transmission of torque from the engine is subjected to an increased load which is also transferred to the case. If the rubber part of the support is sagged or has tears, it does not absorb the shock loads, and the central part is in contact with the outer one. Then the shock goes to the body, which is felt as a knock in the box when you move off.
- The driveshaft cross. Backlashes in the driveshaft arise due to the wear and tear of its moving parts: forks, caps of the cross or their bearings. Because of this, the torque on the driveshaft is transmitted jerkily, not smoothly, which manifests itself as a ringing knock of the cardan when you move off. To diagnose it, you need to manually rotate the shaft on a pit or hoist, setting it in neutral, listening to its sounds and making sure that there is no backlash. Already destroyed cross shaft gives itself a hum and vibration of the cardan on the move, if the symptoms are confirmed, it must be replaced by a serviceable one.
- Cardan coupling. Damper coupling, designed to provide a smooth transfer of torque and additional flexibility of the driveshaft, when worn or torn rubber part does not dampen the shock loads. The stud bushings can come into contact with each other, causing the driveshaft to knock when starting. To check, inspect the coupling, paying attention to its rubber part: there should be no tears, peeling, and the whole part should be flat and retain the factory shape.
- CV joints. The knocking of CV joints on front-drive vehicles occurs due to the wear-out of moving parts of internal joints, located closer to the gearbox, while the external ones usually start crunching on the turns. Knocks are also possible during gear shifting and engine braking and/or acceleration after it. The CV joints should be inspected for the integrity of the dusters and the presence of grease. You should try to rotate driveshafts back and forth by hand, having driven the car to the pit or a trestle.
Why clunking noise sound when shifting gears
- Differential. Wear on the axle and/or pinions, which hold satellites, may cause differential backlash. In this case, on the front-wheel drive, there is a knocking in the box when starting, and on the rear-wheel drive, there is a knocking of the axle when driving off. To check the pin and the satellites without disassembly, you need to suspend the axle first on one side and then on both sides. When one wheel is out — you should try to rotate it. Resistance to rotation should be uniform, and from the differential should not be heard grinding and other extraneous sounds. When the axle is fully suspended, the problem is indicated by a knocking noise from the differential when cranking the wheels both to one side and in the opposite direction. If one wheel rotates by hand, the other wheel should normally rotate freely in the opposite direction.
- Variator. Variator knocking at startup can be caused by worn pulleys, belt or chain. It is possible to detect the problem only by disassembly and defectoscopy.
A knocking noise in an automatic transmission can be caused by overheating, low oil level, or use of the wrong fluid. This causes the friction clutches to malfunction and the gears to engage too hard. If after changing the oil in this case, the problem does not go away.
Sometimes similar symptoms are observed with different malfunctions. For example, a knock when moving backward can be a consequence of wear of the drives, rear engine cushion or gearbox. If the place of occurrence of an extraneous sound is not clearly traced — you need to carefully inspect the entire running gear and transmission.
Is it possible to drive a car that knocks when moving off?
Since the single knock on startup can be a consequence of different causes, the possible consequences of ignoring the problem are also different. The nature of the sound also matters. If you hear a knock when starting off sharply, but there is no knock during a smooth startup, most likely, the wear of the part is minimal and an urgent repair is not required. When the sound is distinct and constant — you must promptly look for and eliminate the cause.
In the table below, the causes of single knocking when starting off are sorted into groups according to the risk of consequences of operating the vehicle with faults in the respective components.
| The vehicle is not roadworthy until the problem is corrected | Urgent repair or unit replacement is needed | It is desirable to fix it as soon as possible |
|---|---|---|
| Engine mount is broken (bushing is torn off, bracket is broken) | Worn gearbox or internal combustion engine support (without destruction of bracket or rubber bushing) | Engine guards knocking |
| Bowl looseness of steering link rod joints | Cardan coupling or cross shaft is worn (without tearing or metal destruction) | Knocking actuators |
| Worn ball joints | Knocking transmission or gearbox differential | Exhaust knock |
| Brake pads cracked | Crunch or knocking in CV joints | Worn clutch disc springs |
| Rupture of cardan joints, destruction of cross shaft bearings | Wear of silent blocks | |
| Loose wheel fasteners | Worn rack supports | |
| Loose hub tighteners |
Weaknesses that often knock when starting on popular cars
Some car models have design features and flaws that often cause knocking when starting off. The most likely causes of knocking at startup on popular cars are collected in the table.
| Car | Probable cause of knocking on startup | How to Diagnose |
|---|---|---|
| Renault Duster | On cars with ABS and ESP, a few slight knocks when starting off are a consequence of the self-test and are the norm. The knocking also often occurs due to the loosening or deformation of the protection. | Inspect the engine guard and check the fasteners for tightness. |
| Renault Logan | On Logan, knocking on startup is often due to wear on the CV joints. It is also possible that the silent blocks of the levers and engine mounts are worn. | Inspect the silent blocks and CV joints. Check the drives for play by rocking and twisting in different directions. Check the silent blocks by rocking the suspension or prying them with a crowbar. Check the condition of the engine pads by squeezing with a mounting spatula. |
| Ford Focus | The weak point of the Ford Focus is the attachment of the drive to the hub. | Check the tightness of the bolt that holds the actuator in the hub. To solve the problem it is possible to fix the splines of a half axle in a hub with thread locker, and then tighten the bolt with 35 N-m torque (for FF2) and pull it through 90 degrees more. |
| Nissan Almera | Knocking, caused by ABS and ESP self-testing, which appear after parking, is the norm. Also, the wear of the CV joints is also possible. | Inspect CV joints, check them for play by trying to twist and rock them. |
| Audi A6 C6 | The main culprit of knocking at startup is worn engine and gearbox mounts. | Inspect cushions, check the amount of their sagging. |
| Mitsubishi Outlander | A knock at startup on Outlander can be a sound of all-wheel drive clutch engagement, in which case it is normal. Self-diagnosis of ABS and ESP is also possible (happens after parking once), which is not a malfunction. From the knocking caused by breakdowns, there is wear of the engine and gearbox pads. | Inspect the powertrain mounts, check their travels while creating force with a tool. |
FAQ:
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Why Does My car clunk when pulling away?
It can be caused by steering tips, ball bearings, hub bearings, engine protection or by loose fasteners.
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What causes a knocking noise when driving?
If you hear a knocking noise when driving, it could tell that the rod bearings are worn out or you have problems with the timing belt.
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What causes clunking sound in front end?
In most cases a clunking sound in front end is produced by suspension struts or worn out control arms.
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Why does knocking in the car only occur when starting off?
Knocking occurs when starting the car because at the moment of transferring rotation from the engine to the wheels, many assemblies are subjected to increased loads.
