Piston Knocking Noise

Piston pin
Piston pin, piston, and connecting rod noises are hard to separate. A loose piston pin, for example, causes a sharp double knock usually heard when the engine is idling, or during sudden acceleration then deceleration of the engine. A piston pin that has been improperly fitted will emit a light ticking noise that is more noticeable with no load on the engine.

Piston slap
Excessive piston-to-cylinder bore clearance will cause piston slap noise. The noise Is similar to a metallic knock, as if the piston were "slapping" the cylinder wall during its stroke. As with most engine noises, understanding the cause of the noise will help you imagine what the noise sounds like. An indication of piston slap is a decrease in noise as the engine warms up. When the engine is cold, the piston to bore clearance is greater and piston slap will be louder. Causes of piston noises include:

- Worn or loose piston pin or bushing.
- Improper fit of pin.
- Excessive piston-to-cylinder bore clearance (piston slap).
- Lack of lubrication.
- Carbon deposits on top of piston striking cylinder head.
- Worn or broken piston ring land.
- Broken or cracked piston.
- Misaligned connecting rods.
- Worn or damaged rings.
- Worn or out-of-round cylinder walls.
- Excessive ring land clearance.
- Insufficient ring-end gap clearance.
- Piston 180 degrees out of position.
- Incorrect piston cam grind.