DS did/ does this too. I read somewhere it can be because having teeth is a new sensation and they're just feeling them out and getting used to them. I have noticed that in the last few weeks DS has started to do it again and he is teething again too so your probably right in that the pressure helps relieve the pain.

It's the most horrible sound isn't it. I cant stand it. It doesn't seem to have done any damage either so I dont think it's anything to worry about, just really annoying. lol good luck, I hope it doesn't last too long.