Engine knock can be caused by many different things. Rapid, uncontrolled combustion of the air-fuel mixture in the cylinder that results in a hard, rattling sound. Detonation can cause severe engine damage if left unchecked for long.

A steady engine knock that is consistent with engine speed- RPM, can be caused by a piston rod failing, a piston rod bearing or a main crankshaft bearing going bad.

Another cause is a piston that is hitting the cylinder wall because it is worn out. This is sometimes called piston slap. Knocking is different from ticking. Ticking is usually a noise that occurs in the top end of the motor valvetrain- rocker arms, push rods or valves.