Clutching at straws time! Did you run the motor by itself - disconnected? You like(?) stripping these drives down? Worth swapping the motor without, then with, parts of the assembly from your quiet drives? You say you've checked the gears. Is any sound a tic-tic-tic-... or a grind... grind... grind...? Hang on, there's a comedy sketch on a TV show I've seen that carries on from here!!
Jason