What's that ticking sound we hear after we switch off the engine.-Mechanical Engineering.

Parts of machine are made up of different metals which are perfectly joined (or constrained) to each other. When engine works, heat is generated resulting in expansion of these metal parts. When engine is turned off, parts of different kind of metals cool at different rates that is a part contracts with respect to its counter part. Thus, they temporarily develop stress in their joints resulting in the tickling sound.