The rotation noise in one direction is from the torque of the motor, especially on the worm gear designs. The force on the gear changes with motor direction. Adjusting for gear mesh or generously lubing may help. Of all my tanks, only the IV is it noticeable since there's no sound on rotation.

I agree, it's annoying, but a metal motor housing in a metal bracket connected to a plastic box is a nice amplifier for certain noises.