SlowCo
1 MW
The gears meshing at higher rpm's will probably be the main source of the noise. If the main gear could be coated in rubber everywhere where it doesn't touch anything (teeth, bearing etc.) that might help. And if there is a surface of the main gear that has more room between it and the casing then gluing a thicker piece of rubber sheet/ring to it might help even more with dampening the high frequency vibration. Sticking some rubber on the outside of the casing and where it mounts against the frame is an easy way to try if the sound box effect of it can be reduced. Not too much as that might cause it to overheat.