Still call up Apple or take it to a store, it'll give you peace of mind you won't find here. People will just speculate as we can't actually see it.
For what it's worth, and from what you've described, I'd say it's possibly the magnet in the display (Which is used to tell the computer it's closed, and so go to sleep). If you take it to Apple, they'll at least be able to diagnose it for you and if need be repair it.