Why are some mendi's quiet and some so noisy. Is there anything that can be done about this?
The answers so far have been no. I'm Having a problem excepting this. If some are quiet I would
think they all could be made that way.

Buy some headsets and get XM...

"Why are some mendi's quiet and some so noisy."
The reason some Mendeola's are noisier than others is a mystery.
The factors that lead to this are the key to this symptoms.
Fact 1) nose on deceleration in one particular gear. this gear is usually a coarse pitch gear with minimal helical angle. change gear to different ratio with more helical angle.

Fact 2) noise on deceleration and acceleration in one particular gear. again, this gear is usually a coarse pitch gear with minimal helical angle. change gear to different ratio with more helical angle. (opt 2)This gear has been in the gearbox for an period of time and has just started this noise, than this gear has been given an excessive duty cycle and needs replacement.

Fact 3) noise on deceleration and, or acceleration in all gear selections. this is based around ring and pinion set up or wear in. examine pattern on ring and pinion, readjust ring and pinion.(opt 2) oil to lite in viscosity, break down in oil due to heat flashing,improper break in procedure at initial running of gearbox.

above are facts related to gears only. bearings tend to make particular sounds of there own. this is why it is key to not only visually inspect the vehicle, but audibly inspect the vehicle.