The sound of knocking (as on a door or in an engine or bearing)

In spark-ignition internal combustion engines, knocking occurs when combustion of some of the air/fuel mixture in the cylinder does not result from propagation of the flame front ignited by the spark plug, but when one or more pockets of air/fuel mixture explode outside the envelope of the normal combustion front.

Knocking is a 2006 documentary film directed by Joel Engardio and Tom Shepard that focuses on the civil liberties fought for by Jehovah's Witnesses.

Knocking is a 2021 Swedish thriller film directed by Frida Kempff and written by Emma Broström, based on a short story by Johan Theorin.