Gear noise is generated by tooth-to-tooth load transfer that causes pressure-pulse trains radiating through the gearset and motor housing. The noise’s frequency is the product of gear rotational speed and the number of gear teeth. Most types of gear noise occur at tooth mesh frequency or harmonics in the audible range.
Gearbox noise can be very annoying — even when it isn’t the most significant noise source. That’s because it occurs as pure tones that the human ear can detect even at 10 dB lower than the overall noise level.