O.K.
I suspect that the reason that you are not getting a lot of responses to this is because gear noises are caused by just about any part of the entire drive assembly. A lot of times, the gears aren't the cause of the problem. Resonance can be a major factor. Sometimes the box itself is resonating.
If the gears are out of balance, or if the ratio happens to be a harmonic of any of the other turning / vibrating / pulsing parts. . ., all of these can be causing problems. The American Gear Manufacturers Association (AGMA) has a specification on gear noise. If you contact them, you can probably purchase a copy from them. It may provide you with some insight.