These are photos taken during teardown of the rear part of the transmission of my Deckel FP2NC milling machine.  The gears whine loudly on certain speed settings.  I've traced the whine to gear tooth wear.  The wear is very subtle, which serves to illustrate how little geometric deviation is needed to make gears noisy, and also serves to illustrate how precise spur gears must be to be quiet.  On speed settings that don't use the worn gear set, the machine sounds like a Monarch EE despite being gear driven.