RE: Gearbox noise: silencing?
Well... brass gears are noisy.
That doesn't mean they cannot be silenced but they are inherently noisy.
Okay, besides cleaning the gear-faces, have you ran them in any solution or oil? I would set them in some oil and run them with a battery for quite a few hours and then see what you come up with.
This is probably the best way to "run in" the gears.
~ Jeff