noise can be generated from a lot of sources in a gear drive
listed are some sources of noise
1 involute profile variation
2 helix angle variation
3 tooth to tooth error
4 misalighment of the bores
5 noise from bearings
6 wrong design of the casing
7 deflection of gear box shafts during load generating noise on the gear drive
We try to limit the composite noise level to about 70 decibel on a gear drive
Reference "Noise Control - Measurement,Analysis and Control of Sound and Vibration" by C.E.Wilson ISBN 0-89464-878-9
As well as all the other "mechanical" influences discussed by other contributors, the tips of the teeth of large high speed gears rotating at 30,000 rpm can be exceeding the speed of sound - yet another source of significant noise generation.