Engine knocking is the sound produced in an internal combustion engine when the fuel does not ignite properly. The mixture of petrol and air is meant to be ignited only by the spark plug at the right moment in the stroke of the piston in the engine. If there is a problem with the engine, this fuel and air mixture can be ignited at the wrong time by pressure and heat. The explosion of the mixture makes a destructive knocking sound. This uncontrolled explosion can damage the engine and may even destroy it.