Actually, today I took mine in for an odd vibration issue. I know it sounds weird but instead of the "normal" noise it makes when it vibrates, it had a weird "buzz". If you pressed on the back glass it would stop. I took it to my local Apple store and the genius that looked at it couldn't figure it out. He took it around to other Apple dudes and non of them could figure it out. They took the backplate off and checked to see if anything was loose, nothing was. In the end, they ended up replacing it. Its a minute issue, but like i told the apple dude, it just bothered me because I knew it wasn't right. I am not OCD but I do like my things to work as advertised.
Try taking it in, it never hurts.