There's a semi-metallic sort of rattle, like a high-frequency vibration coming from the passenger door area of my '09 R55. It just appeared one day and can be very loud when going over bumps, as if it's coming from the entire door!

A few weeks ago I added a CLD (like Dynamat) to the interior door skin and panels, requiring removal of the interior trim piece and the whole door panel (except the window mechanism, which I worked around). There was no rattle, then it suddenly appeared a few weeks later!

So I went back in there, checked everything I could think of, and saw nothing unusual. Taped down any loose wires I could see, and still... the rattle!

What did I miss or forget to check? Any ideas what this could be? Could the window be vibrating in the mechanism?

edit: I would take this to the dealer, but... I don't know if they'd cover it after I swapped out the speakers and took the doors apart