Sound decoders - How quiet should a motor be?

[Traintresta]

I've been away from DCC for about five years but fully intend to add DCC and sound fitted loco's to the layout I am currently building.  I'm currently debating the future of an old Atlas/Kato (Yellowbox) RS-3, as the motor has quite a loud whirring noise and there is a little grinding sound coming from one of the trucks, but I cannot find anything wrong!  I have serviced the loco and applied small amounts of silicone grease on the gear and worm screws, and a very tiny touch of oil into the motor bearings but nothing has changed.

 

My question is, how noisy a motor can you hide with DCC sound?  I've always admired quieter motors and previous DCC installs were on relatively quiet loco's so I don't suppose I've ever thought about this before, but the question is will that noise be heard over the sound of a decoder?  Now I know there's always the choice of volume to factor into this so I'll say that I do not intend to have the loco's particularly loud as I do find lots of noise becomes uncomfortable but I do like prime mover sounds as a loco moves back and forth.

 

Has anyone got any thoughts or experience on this issue?