http://www.e90post.com/forums/showthread.php?t=673146
http://www.e90post.com/forums/showthread.php?t=442607
That's just a couple of them, there's more trust me. Don't know what it is, but myself and many others experience this. On new and old N52s. I don't know if I can call it normal since other people don't experience it, but I would say don't worry about it.