Vehicle noises are very difficult to identify with worded descriptions. What you end up typically are a bunch of haphazard guesses. We could always go through a process of elimination, but the best thing to do is take it to a BMW tech and let him/her take a look and a listen.

When the transmission is in "park" and you rev the engine, does it make the same sound? Or does it occur only when the vehicle is moving? Are you able to identify the general area where the sound is coming from? That might help narrow it down between engine or tranny. Does it sound like the movement of air, i.e. a vacuum leak?