I am following this thread with a lot of interest, thanks for it!
I have also the same noise: only noticeable with the roof closed and on very bumpy roads.
I didn't have time to take a look at it because I only hear it when I don't have music on on the bumpy section of my way to work, so not so often.
But I am very interested if you find anything for it, ManiacGT
(You would save me time and head banging!)