I have a similar sounding problem with an '87 turbo that was converted to a manual, but its in 3rd and 4th gears when the rpm's are below 2000 and the drive line is under some amount of torque (doesn't happen when sort of coasting down hill). No drone if rpm's are kept above 2000.

New flex discs were installed with the tranny conversion. After racking my brain and checking a bunch of stuff, I've decided to check for worn bushings on the shifter as the next possibility as those weren't addressed with the swap. Donor car for the tranny had 180K and the tranny has a bit of noise when in neutral which appears to be well documented in this forum.

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