'07 Manual X3 Rear Vibration/Humming

My ride has developed a humming/vibration sound that seems to be emanating from the rear. This is the problem:

1) I only hear it shortly after starting.
2) I only hear it with the clutch DIS-engaged. (so....what could actually be moving in the back?)
3) It's NOT the fuel pump etc. Doesn't sound like that at all.
4) It goes away after driving and the vehicle warms up.
5) It sounds like a panel or trim is loose, although I've looked at everything and can't find a source.

Thanks again for all of the help on this blog.

Cheers!

Maybe it's the exhaust valve thing that people use a Golf Tee to modify? It coul be that it's staying slightly open when you start the car so that what you're hearing is a noticable change in the exhaust note. Also, that change in the note (which is deeper) could be causing just enough noise to rattle your license plate??? Not sure but that's a thought I guess. Other than that I'm clueless.

My first thought was the exhaust valve too. But I don't know if that would be it since it is only when the clutch is disengaged.

The clutch thing might be a random thing I've assigned importance to. I will check into the exhaust valve thoughts. Thanks much