The exact same thing happens to me at night when I'm trying to sleep. I've noticed it happens when I'm really tired/stressed/anxious. I looked online and the only thing I can find in relation to it is possible tinnitus, is your hearing affected? I suffer from ear infections and I work in a club so it's loud music all the time, I was thinking it could be this however on all of the websites it says if its tinnitus it's normally a ringing noise whereas with this it sounds like metal banging together in my head and I get a pain in the back of my head.... I'm thinking I'm going to go to the Doctors!