What does engine piston slap sound like?

[William]

Engine piston slap produces a crisp, rhythmic metallic knocking sound inside the cylinder. Here are the reasons for engine piston slap: 1. Severe piston knocking noise occurs during cold starts, which diminishes after the engine warms up. This is caused by excessive piston clearance or piston deformation. When cold, the piston clearance is larger, resulting in more pronounced knocking. As the engine temperature rises, the piston expands, reducing the clearance and consequently decreasing the knocking noise. 2. Carbon deposits in the combustion chamber. Combustion in the engine generates carbon deposits that accumulate on the piston top, increasing the compression ratio and causing knocking.