Knocking when accelerating
Where the noise comes from
the wheels and everything turning with them
Go to the garage without delay
A knock that appears exactly when you press the throttle or lift off usually comes not from the engine itself but from the chain that carries the drive to the wheels. With age, play — free movement — builds up in its parts: the rubber engine mounts sag and let the engine jerk when the load changes, the inner CV joint wears (the joint on the shaft from the gearbox to the wheel), the propshaft universal joints and the subframe mountings work loose. Every time the drive changes direction, the slack is taken up with an impact — hence a single knock or clunk.
The automatic gearbox is a story of its own: a jolt with a clunk when moving the selector between D and R, or during shifts, usually means old fluid or wear. And an entirely different case is a dull knock from deep inside the engine that speeds up with revs and grows louder under load: that is how the bearings on the crankshaft knock. It is rare, but it is the most serious version, and it must not be missed.
Possible causes
| Cause | How likely | Danger |
|---|---|---|
| Engine mounts or the inner CV joint | Most often — a single knock on and off the throttle | shop this week |
| Play in the drivetrain: propshaft joints, subframe mountings | Common on rear- and four-wheel drive — a clunk under the floor when moving off | shop this week |
| Automatic gearbox: old fluid or wear | If the jolt and clunk coincide with shifts | shop this week |
| Bearing knock from deep in the engine | Rare — speeds up with revs, louder under load | pull over |
Can you keep driving
With most of the causes in the table you can drive: play in mounts, joints or the drivetrain will not immobilise the car suddenly while the knock stays single and quiet. Still, do not put the check off for months: a worn universal joint or a tired CV joint eventually breaks up, and that happens while you are driving. A sensible window for the garage is a week or two; until then, move off and use the throttle more gently.
The exception is a knock from deep inside the engine that speeds up when you blip the throttle and grows under load. With that, ordinary trips have to stop: worn bearings can end in a seized engine. Check the oil level first; after that, a tow truck — or, if the garage is very close, slowly and off the throttle.
What to check yourself
- Pin down the moment it appears: a knock exactly on and off the throttle rather than over bumps is about the drive being transmitted, not about the suspension. That detail narrows the mechanic’s search straight away.
- Check the engine oil level on the dipstick. With any knock that sounds like the engine this comes first: with a low level, the bearings suffer before anything else.
- If it clunks during shifts, check the level and condition of the automatic gearbox fluid: dark fluid that smells burnt is a frequent cause of jolts, and sometimes changing it settles the matter.
- Listen to where the sound comes from — under the bonnet, from under the middle of the floor or from the side of a wheel — and whether it repeats when you change gear. These observations are worth writing down for the garage.
- Do the gentle test: if a soft press and release of the throttle produces no knock while a sharp one brings it back, that is classic play, and gentle driving is enough until the repair.
Narrow the cause down with questions
Answer a few questions and the diagnostic tree will shorten the list of causes for your case.
How the app helps
The Pro-Stuk app asks the same follow-up questions — when exactly it knocks, whether the sound changes with revs and gears — and the recording helps tell the clunk of play from a deep engine knock. The report gives likely causes with percentages and a plain conclusion: drive on, plan a garage visit, or stop.
Common questions
Why does it knock exactly when I press and release the throttle?
When the load changes, the drive reverses direction and all the clearances in the engine mounts, joints and drivetrain are taken up with an impact. While the car cruises evenly the parts are pressed together and the play does not show itself — which is why the knock is heard only when you work the throttle.
How do I know it is the engine itself and that it is serious?
The warning sign is a dull knock from deep in the engine that speeds up with the revs and grows louder under load, uphill for example. That is how crankshaft bearings knock. In that case stop driving, check the oil level and get the car to a garage on a tow truck.
Can an automatic gearbox cause knocking under acceleration?
Yes. A jolt or clunk at the moment of a shift, or when moving the selector between D and R, is a typical sign of old fluid or a worn gearbox. Start by checking the level and condition of the gearbox fluid; until the repair, move the selector only at a complete stop with the brake pressed.
The app for iPhone and Android
The questionnaire, the sound recording and the full report with probabilities are all in the Pro-Stuk app.
Installed from a store, the app updates itself.