Chitra Narayanan, Facebook asked:
Why do doors creak?
Dave - If you have a door which is getting a bit old, getting a bit rusty and you haven’t oiled it properly, you get a very similar thing to brakes squealing or in fact, dragging finger nails down a blackboard. It’s to do with stick and slip. What happens is that you get quite a lot of friction and there's a lot more friction when two things are stationary than when they're moving. So, essentially the two bits of the hinge lock together, then the hinge bends a bit, and then the force gets enough for it to break, and it jumps forward a bit and then it stops, and then it bends a bit, and it jumps forward…
This jumping can happen at a range of pitches, including the horrible 50-100 Hertz range. As you’re getting jump, jump, jump, jump, what you hear is this horrible creak noise.
For the technically inclined, it's an example of hysteresis which shows up in a lot of dynamic situations. Geezer, Fri, 4th May 2012
Slip-stick is the mechanism for making annoying noises by running your finger around the rim of a wineglass I believe. I presume that stringed instruments (arco) also work like this imatfaal, Fri, 4th May 2012