My M6 makes a rumbling sound and feel in third and fifth gears only. Worse with power, and sometimes the fifth gear sound will change (improve) with reapplication of the clutch. It has been doing this for several thousand miles with no change, although I tend to not use 3rd and 5th nearly as much.

I have stock tranny mounts, LS6 Corvette clutch with 85k on it, and Moser 12 bolt.

Any thoughts?