There are probably a couple items in your comments that make noise while in reverse.
First off, hypoid gears, those that are in a standard differential tend be quiet when run in forward and sometimes get a bit noisy when run in reverse.
Also most older cars have straight cut gears in reverse and low gear. So a bit of whine in low or reverse was normal. Some had straight cut on all forward and reverse gears. This caused some noisy when moving in any direction.
Harley in their awesome wisdom in '07 when they came out with their 6 speed Cruise Drive transmission, they had 1st and 5th as straight cut gears. Due to the gearing of the primary and that the bike at anything less than 70 mph would not pull itself in 5th gear, most people ran in 5th most of the time. Boy did they make noise...
That's when people started trying lots of different trans lube. The prevailing winner was Red Line Heavy Duty Shockproof. That's why it was a lube that I thought might help quiet down the M3W BB.
In '10 or '11 they made 5th gear a helical cut gear and that removed the whine in 5th. My '09 still has the straight cut 5th and with Red Line Shockproof, I can hardly tell the difference in noise between 4th, 5th or 6th.
