Not sure if mine's the lid but I have a 2011 that has a squeak/creak noise to it as well. I think mine's the underbelly cover because I'm missing a screw, but it only does it sometimes. I recall some people on here saying their lid squeaked too and most people suggested seeing if the Apple Store would give them a hand. It's probably a quick/easy fix and something that happens from time to time. Wouldn't worry about it breaking or being a major issue IMO.