system
Given the following post, respond as if you're commenting on Reddit:
human
What is the thermodynamic process that makes hot metal "tick" as it cools? 

I understand the basic concept of materials expanding and contracting as they heat up and cool, but I do not experience an audible signature of this process commonly except when a hot car begins to cool rapidly. Is it the level of heat? The type of metal or material? How rapidly such materials cool? 
Thank you Askscience!
gpt
You hear stoves and car engines ticking as they cool (and as they heat up, actually), as the result of metal expanding or contracting due to the heat.  Specifically, the ticking sound originates at seams between pieces of metal that are bolted or otherwise fastened without being welded together.  There is static friction at these points, and, as the metal expands or shrinks, the friction gives way briefly, resulting in the ticking sound you hear.  That's also why you don't hear a muffin tin or cookie sheet ticking after you remove it from the oven - there are no seams.

You can make the same basic thing happen by pressing your finger hard on a surface and attempting to slide it.  Instead of starting to slide smoothly, your finger pops out of place, jumping to the next spot and stopping before the force of your push makes it overcome the friction to pop out of place again.  Nails on chalkboards, squealing tires, a bow being drawn across a violin's strings and a cooling car engine or wood stove all rely on static friction to make their sounds in this way.