A metallic rattling sound produced by the engine during heavy acceleration when the ignition timing is too far advanced for the grade of fuel being burned. The noise is caused by vibrations in the cylinder walls , head , and piston . When an engine pings, the normal, controlled even spread of the flame front is disrupted by the spontaneous combustion of pockets of fuel. Their collision sets off the pressure waves that result in the pinging sound. Pinging can be caused by bad timing , inadequate octane rating, incorrect fuel-air mixture , a hot spot in the combustion chamber caused by a glowing piece of carbon , or an overheat condition. Also called "knocking ." Also see detonation .