An abnormal engine noise occurring when excessive clearance between the piston skirt and the cylinder bore allows the piston to tilt or rock as it moves. This metal-on-metal contact creates a distinct hollow knocking or rattling sound. It is most prominent on cold starts when parts have not yet expanded from heat, and often quietens down as the engine reaches operating temperature.
Heard on the job
That high-mileage LS motor has some serious piston slap when you first fire it up in the morning.
Insider tip
While annoying, minor cold-start piston slap is considered normal wear on certain engine families and does not always indicate imminent failure.