(Otomotiv) A sound that occurs in the cylinders when the fuel/air mixture is ignited too soon and the subsequent explosion hits the piston as it travels up the cylinder on the compression stroke. Usually, it's due to faulty timing, low- octane gas, or fragments of burning carbon in the cylinders. Also called pinging or pre-ignition. It sounds like marbles rattling in a can and can be heard best when accelerating up hills. This sound can also be caused by impending mechanical failure within the engine due to extreme wear of a piston or bearing. See also four-stroke power cycle