Unusual sounds from the diesel engine: knocking, tapping, rattling, or grinding. Could be valve clearance out of adjustment (tapping), worn main or rod bearings (deep knocking), piston slap (slapping at cold), or turbo noise. Engine noises are the most serious category.

What you'll see

Unusual noise clearly coming from the engine, not the compressor or panels. The character of the noise indicates the source: Light tapping from the top of the engine = valve clearance or rocker arm. Deep knocking from the bottom = main or rod bearings (serious). Rattling that changes with RPM = timing chain/gear or accessory drive. Screeching/whining = turbo bearing failure. Slapping from the block that is worse when cold = piston slap (minor on cold start, serious if persistent when warm).
Before you assume this is the problem

Confirm the noise is from the engine using a stethoscope. Air-end bearing noise, coupling wear, and loose panels can all be mistaken for engine noise. See: Worn Air-End Bearings, Coupling Wear, Loose Panels.

See all causes of unusual noise / vibration →

How to diagnose

  1. Characterize the noise

    Light tapping that is consistent with engine speed = likely valve clearance. This is the most common and least serious engine noise. Check valve clearances per the maintenance schedule (typically every 1,000 hours). Deep, rhythmic knocking = bearing wear. This is serious. Turbo whine = see Turbocharger Problem.
    Result: Noise character helps identify the source and severity.

  2. Check valve clearances

    If the noise is a light tapping, check valve clearances with feeler gauges. Set the engine to TDC on the compression stroke for each cylinder and measure the gap between the rocker arm and valve stem. Compare to engine specification. Incorrect clearances cause tapping noise and affect engine performance.
    Result: Out-of-spec clearances = adjust. In-spec = noise from elsewhere.

How to fix it

  1. Adjust valve clearances

    Set valve clearances to the engine manufacturer specification using feeler gauges. This is a standard maintenance item that should be done every 1,000 hours. It resolves tapping noise and restores optimal engine performance. Typical clearances are 0.15-0.30mm for inlet and 0.25-0.40mm for exhaust, but check the manual.

  2. For serious engine noise, seek specialist help

    Deep knocking, grinding, or rattling that is not valve-related indicates internal engine wear (bearings, timing components, pistons). This requires a diesel mechanic with the tools and knowledge for engine disassembly and measurement. Do not continue running an engine with a heavy knock -- the cost of prevention is far less than the cost of catastrophic failure.

Common mistakes

Do not ignore valve clearance maintenance. It seems minor but incorrect valve clearances affect combustion efficiency, fuel consumption, emissions, and can lead to burned valves (if too tight) or broken rocker arms (if too loose). It takes 30-60 minutes and costs nothing but a feeler gauge and patience. Also: do not confuse normal diesel engine noise (diesels are louder than petrol engines by nature) with abnormal noise. Learn what your engine sounds like when healthy so you can detect changes.

Parts & tools

Feeler gauge set. Wrenches for rocker arm adjustment nuts. Mechanics stethoscope for pinpointing noise. For major engine work: diesel mechanic required.

Review safety precautions before starting →

Safety

Deep engine knocking is a warning of imminent bearing failure. If you hear a heavy knock that was not there before, shut down immediately and investigate. Continued running risks catastrophic engine failure (thrown rod, cracked block).

This issue can also cause