Why does metal squeak?

[Grim Reaper 6]

It turns out, it's all about the periodic shift between metal pieces sticking and slipping against one another. Metal's stiffness and density also make that squeak extra loud, experts told Live Science.

The mechanism behind metal squeaking is no different from the squeal of skidding tires or the squeak of a floorboard, Robert Hyers, a professor of mechanical and materials engineering at Worcester Polytechnic Institute in Massachusetts, told Live Science in an email..

"The squeaks are really periodic thumps," Hyers said. "If the thumps are close enough together, you perceive the high-frequency thumps as squeaks." In other words, when these thumps happen in quick succession, they emit a high-pitched squeaking sound..

https://www.livescience.com/chemistry/why-does-metal-squeak