How quiet do you want it, and how far are you willing to go to get that quiet? Simply changing to a machined spur (like a JConcepts Silent Speed) and a machined pinion (like a Robinson Absolute) will quiet it down quite a bit from stock. Then you can go much further-- run the plastic ringed diff, clean-up, deburr, bevel, and polish the idler and topshaft gears, etc, etc.
There are no atheists in the foxholes / There is no intellect in the air / There are no scientists on the way down / Just a working example of faith versus physics. --El-P, Flyentology