Distinguishing Engine Clunk From Other Clunks
The word "clunk" describes a heavy, single or irregular thudding sound, distinct from the rhythmic ticking or knocking of bearing wear. An engine clunk is often related to movement — of the engine itself, of accessory components, or of loose brackets — rather than continuous internal mechanical action.
That said, a heavy irregular clunk from deep inside a running engine is one of the most serious sounds a car can make.
Engine Mount Clunk
By far the most common cause of what drivers describe as an "engine clunk." The engine is held in the engine bay by rubber-and-metal mounts (usually 2–4 of them). These mounts dampen vibration and allow controlled movement. When a mount tears, collapses, or the rubber separates:
- The engine moves excessively during acceleration and braking
- A clunk or thud is felt and heard as the engine reaches the limit of its travel and strikes a crossmember or bracket
- The noise is often worst when pulling away from a stop, changing gear, or going over bumps
Engine mounts are a relatively common repair — particularly on Japanese imports with over 100,000 km. Replacement cost: $150–$500 NZD per mount, fitted. Most vehicles have two or three mounts.
Accessory or Bracket Clunk
Loose alternator brackets, AC compressor brackets, or power steering pump mounts can clunk as the components shift under load. Usually a rhythmic thud that varies with RPM rather than driving events.
Internal Engine Clunk (Serious)
A heavy, infrequent clunk from deep within the engine — sometimes felt through the seat or floor — can indicate:
- A broken or collapsed piston (very serious — engine damage)
- A dropped valve (requires valve to have broken or seat to have failed)
- A severely worn main or rod bearing about to fail completely
- A broken connecting rod (which may have already damaged the block)
None of these internal clunks are "wait and see" situations. If you hear a new heavy clunk from within the engine itself (not from outside the block), stop the car safely as soon as possible and have it towed.
How to Narrow It Down
- Only happens over bumps or kerbs → engine mount or bracket
- Worse on acceleration or gear changes → engine mount
- Random, from deep inside, at any RPM → internal engine fault (urgent)
- Accompanied by sudden power loss → major internal failure
A mechanic can confirm engine mount wear by placing the vehicle on a hoist and checking mount condition visually and by feel.