While we are in full blown speculation mode I would to throw another theory into the pot.
We all know the drive is very lumpy with big torque spikes many thousands of times a second at speed.
With the compensator not acting as a Cush drive ( another story ) the poor old bevel box is getting hammer blows on the input in audio frequencies.
So I my theory is the bevel gears are bouncing off each other with each torque pulse so making the noise we love and hate. There will be many factors which will make this better / worse which is why we see so much variation : like belt tension , oil temperature , compensator wear, rear wheel pulley wear, Inclusion of NVH kit etc etc
Just a theory
