A knock that appears or grows under load — on acceleration, uphill, with a full car — is more worrying than a knock at idle. Under load the engine’s parts are pressed together by gas pressure, and if they still manage to knock, the clearances are already serious. That said, not every sound under load means an expensive repair: the commonest cause, detonation from fuel, is cured by changing where you fill up. Here is how to tell the scenarios apart by sound and what to do in each.
Why load reveals a knock
Load is the moment when the driver asks the engine for everything: pedal to the floor, overtaking, a climb, towing. The pressure in the cylinders rises several times over, and with it the forces acting on the pistons, gudgeon pins, con rods and crankshaft. A worn clearance that stays silent at idle starts announcing every power stroke under impact loading.
Hence a simple rule: a knock tied to load more often points at the crank mechanism (pistons, rods, crankshaft and its bearings) or at detonation, while a knock at idle points at the valve drive and the accessories. If yours is the second case, start with an engine knock at idle.
Detonation: ringing on acceleration
Detonation is explosive rather than smooth combustion of the mixture. Drivers describe it as pinking: a bright metallic ticking or ringing on acceleration, especially when labouring at low revs in a high gear. A detailed description of that sound is on the symptom page pinging when accelerating and in signs of engine detonation.
Typical causes, from common to rare:
- fuel with an octane rating below what is required, or poor quality fuel;
- carbon deposits in the combustion chambers, raising the effective compression ratio;
- overheating or pre-ignition from worn spark plugs;
- a faulty knock sensor or problems in the cooling and EGR circuits.
Detonation is destructive not through its sound but through its pressure waves: they break piston ring lands and hammer out bearings. A one-off ring on bad petrol is not a sentence, but regular detonation shortens an engine’s life noticeably. A modern engine defends itself: the knock sensor catches the characteristic vibration and the control unit retards the ignition. The car loses pull and uses more fuel in the process — so if performance dropped and the ringing vanished after filling up somewhere new, that indirectly confirms the fuel theory.
A separate case is cars converted to LPG. Badly set gas equipment leans the mixture, and a lean mixture burns hot and is prone to detonation. If the ringing appeared after a conversion or a re-tune, start the search with the gas system settings rather than with engine mechanics.
A knock from deep down: bearings and the piston group
The most serious scenario is a knock in the crank mechanism.
Big end bearings — the plain bearings in which the rod turns on the crank journal — give a dull, soft knock from deep in the block as they wear. It grows with load and revs and is often clearly heard on a sharp lift-off. Companion signs are falling oil pressure and metal particles in the oil. Driving with that knock is not an option: the next stage is a spun bearing and crankshaft work — what big end bearing knock sounds like.
Main bearings — the ones the crankshaft itself turns in — knock lower and duller, the sound coming from the very bottom of the block: signs of main bearing knock.
A gudgeon pin gives a bright double knock precisely tied to revs — gudgeon pin knock: how to identify it; piston skirts are more often heard when cold and are covered in an engine knock only when cold.
The timing chain and engine mounts
A stretched timing chain slaps and rattles against the guides under load, a rumble can appear on acceleration along with flat spots, and the check engine light often comes on with cam timing codes. The full list of signs is in signs of a stretched timing chain, and the risks of driving in that state in can you drive with a stretched timing chain.
Engine mounts — rubber-metal blocks — give a single dull thump as you pull away, change gear or blip the throttle when they are worn: the engine rocks more than it should and hits its limiter. That is unpleasant but not dangerous. It is easily checked: a helper watches the engine with the bonnet open while the driver, on the brake, moves the selector briefly between D and R — noticeable rocking gives away a dead mount, as in checking engine mounts under load.
Comparing the scenarios
| Sound under load | Likely cause | What to do | Danger |
|---|---|---|---|
| Bright ticking, ringing on acceleration | Detonation | Change filling station, check plugs and sensor | Medium if repeated |
| Dull knock from deep down, grows with revs | Big end or main bearings | Stop driving, recovery truck | Maximum |
| Bright double knock with revs | Gudgeon pin | Diagnosis without delay | High |
| Rumble, slapping at the front, check engine | Timing chain, tensioner | Diagnosis within days | High |
| A single thump pulling away or shifting | Engine mounts | Scheduled replacement | Low |
How the repair is decided
The sensible sequence is cheap and reversible first: fill up at a station you trust and see whether the ringing goes; check the oil level and condition; record the sound; then go for a diagnosis without loading the engine on the way. The general symptom page is knocking in the engine.
What decides the work is which of the three questions the checks answer. Did the ringing survive a tank of correct-octane fuel — if not, it was never mechanical. What does the oil pressure measure at hot idle and at 2000 rpm — that separates bearings from everything else. And what is the cam timing offset on the scan tool — that separates the chain from the crank mechanism. Each answer costs a fraction of the part it rules out.
Two things worth asking. If detonation is diagnosed, whether the knock sensor and the cooling system were actually checked, or only the fuel blamed — a sensor that has stopped reporting lets the engine detonate silently. And if the chain is replaced, that the guides and dampers go in with it: they are the plastic parts the chain beats against.
The difference between “change filling station” and “rebuild the engine” is the difference between an early and a late diagnosis, so a knock under load is not something to sit on.
If you are unsure which scenario yours resembles, record the sound of acceleration in the Pro-Stuk app: it compares the recording with typical knocks, asks a few clarifying questions and shows the likely causes with a level of urgency. With that prepared at home the workshop visit goes faster.
