The sound of a machine designed to produce mechanical energy. Combustion engines burn a fuel to create heat, which then creates a force. Electric motors convert electrical energy into mechanical motion. Other classes of engines include pneumatic motors and clockwork motors.
A metallic "pinging" sound caused by a malfunctioning spark-ignited internal combustion engine. Engine knocking indicates that the combustion of the air/fuel mixture in the cylinder is occurring at the wrong point in the piston's stroke.