Piston slap is exactally what Jeffco stated, although I'd like to add that It occurs in all vehicles, new and old. It only happens in most when the engine is first started, and not had time to warm up, Metal expands as it warms up, and therefore the rings provide a better seal between the bore and piston, which prevents the piston slap. This is why the first few minutes after you start your car are the most crucial.