When an engine's air-fuel mixture combusts, it makes a knocking sound. As the combustion chamber pressure and temperature rise fast, metal components vibrate, creating a shockwave.  Spark plug spacing issues are a common cause of knocking. Always cover your car's cylinders with spark plugs. Due to worn parts or faulty installation, incomplete combustion might cause knocking noises from your vehicle's cylinder walls. Misfires reduce fuel economy! If there is too much compression from carbon buildup on your piston head, high pressures in your cylinders can cause backfires and pre-ignition, which can cause knocking noises from your tailpipe/header pipes when accelerating or decelerating hard (elevated revs). Incorrect fuel mixes can also produce valve clogging and knock sounds from the exhaust and intake manifold vacuum leaks/cracks. It's important not to ignore this issue as it may lead to expensive repairs down the line, so we recommend taking it to an experienced mechanic for inspection, who will likely start by inspecting all ignition components before moving on to likely causes like those mentioned above. Supercharging problems can also cause similar symptoms, so check any performance upgrades you may have installed!