could be belt related or....
there are noise issues related to valve cover, vanos solenoid, eccentric shaft sensor as well. I have a thread "CEL and a whistle" that may or may not be relevant. There is a recent thread in the n52 subforum regarding P0012 error msg, that might be relevant. I would browse /search that forum and the maintenance forum. If it keeps happening, have someone inspect the belt or scan the car for codes.
edit for...
link to CEL and a whistle
http://www.e90post.com/forums/showthread.php?t=691932
link to P0012 error
http://www.e90post.com/forums/showthread.php?t=719660
hopefully it's something simple like a belt or cleaning the vanos solenoids.
note your car may be recording BMW codes even without a check engine light. There is some good windows software out there ... **************.com if you want to check codes yourself. If you have a check engine light, autozone and other such places can read it for free, but they may not be able to see the BMW codes.