I've owned a 13" 2011 Air since it came out. Today it began making an odd, loud popping sound. It's easy to reproduce and it happens every time. If I apply pressure around my left wrist, directly above the rubber foot, it pops upon release. It sounds like metallic contact, or perhaps the release of metallic contact. If this doesn't go away, I will need to take this into the store. It's much more than a creak. Any thoughts on why it's happening? Thanks.