I'm trying to do everything I can do get a reasonably quiet spindle with a belt drive and servo spindle motor. I understand it will never be silent at 5000 RPM but it shouldn't be screaming.

So far the best I've been able to get it is ~95dB at 3500 RPM. My motor isn't setup for 5000 RPM yet but I imagine it would be well over 100dB. This "best so far" setup uses upgraded tapered roller bearings (I had AC bearings and they were worse) and using single V belts with (I think) 4L belt profile. I've also tried J6 poly v belt pulleys and timing pulleys. The poly v is similar to the single v belt but slightly louder I believe. The timing pulleys are the loudest. I'm also using Hoss' 3D printed spline bushing. I'm not sure if it would work better printed in something other than PLA or if I made a bushing out of a different material somehow.

I do have the stock bearings in the quill so my best guess is that those are what's making the excess noise. My other guess is that the stock spindle bearing preload nut is just garbage and it's not holding preload well enough so there is somehow excess noise due to some kind of play?

I found this video where he is using a servo spindle motor and pulleys that aren't properly aligned and it sounds like it's whisper quiet at 3000 RPM. If I had my spindle sounding anything close to this I'd actually want to use my mill!

https://youtu.be/ENFmHKaDIOM?t=162

For comparison's sake, here is my mill at 3500 RPM -

https://youtu.be/QVBZl8imAsA