When my idler pully went bad it whined but it was a lot louder and kinda sounded like a supercharger. Does the wine increase with the throttle? If so you can try it it was only like $70 and took 20 min to change.
Its item 1 here http://www.realoem.com/bmw/showparts...40&hg=11&fg=18
Tisher has them for $86.05