What is knocking of engine?

The knocking of the engine is generaly due to the way the fuel is combusted, if the spark occurs to early or too late in the 4 stroke cycle then the power stroke will be more like an uncontrolled explosion rather than even burning of fuel, this can also occur when the fuel is being ignited from another source such as a (hotspot) in the cylinder, this could be carbon build-up on a valve that stays glowing red and ignites the fuel too early (before the sparkplug is due to spark) - called "detonation"