ELI5; why do cars make little tinking noises right after you turn them off and get out?
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Some metal parts inside the car are very hot when the car is running. When you turn the cool off, those parts cool and equalize with the environment. Since different metals heat and cool by different amounts, this can cause tiny slips. When the metals slip, they make the “tink” sound you hear.
The metal in the engine and surrounding area gets hot when the engine is running (since the engine contains a continuous series of small explosions), and so when you turn it off it cools down to ambient temperature. Since heat causes matter to expand, you’re hearing the noise of the metal shrinking as it cools down.
You can hear the same noises when you use an electric stove – the heating element and the pan will expand with heat and then contract again as they cool, and you can hear audible noises associated with the volume change. It’s very obvious if you take a cold pan and put it on an already hot burner.
The metal parts expand from heat while driving. If you every touch the engine after driving you’ll notice it’s very very hot and can burn you if you aren’t careful. After you turn the car off, it starts to cool down and the metal parts contract and that’s the cause of the noise.
As per the other answers, the metal shrinks as it cools. The other answers haven’t explained how you get a tick sound as a result of that.
Internally, metals have a lattice structure. When a metal is cooling and contracting this sets up some tension in the structure where a layer (and all the layers above it) want to move but they can’t because they’re held in place in the lattice.
After a while the forces build up to the point that the “moving” forces overcome the “staying still” forces and part of the lattice shifts relative to the rest. That sudden movement and stopping is what generates the tick sound.
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