Contraptor

Background
http://www.contraptor.org/forum/t-274259/silenced-motor-mount
http://www.contraptor.org/forum/t-193855/noise-control

Noise has been somewhat of a problem with mini CNC. The machine is fairly quiet at most of the feedrates. However there are a couple of feedrates that result in loud and nasty vibration. In my testing these are 2.1 IPM and to a lesser degree, 4.2 IPM. Any diagonal move that happens to have one of the axes moving at these speeds becomes a problem.

Short version

To reduce the noise, I basically followed Arthur's previous work at the link above and added some degree of vibration isolation between the following:

Leadscrew and bearing
Instead of steel nut, a plastic nut is in contact with the bearing. Not visible in the picture, there is some teflon tape wrapped around the leadscrew where the (ball) bearing is. This prevents leadscrew from rattling around. I added the thrust bearing for reasons other than noise control, but it also has a plastic nut. The combination of plastic nuts with steel nuts should eliminate the need for spring washers (which were skewing the nuts).

Motor and motor mount
The motor is connected to the mount with nylon screws and nuts. There are several neoprene washers in between, as well as a pair of grommets in the mount holes (visible on the right pic). I found that metal screws didn't really work well there. The motor flexes on this mounting, but not in the twisting direction so it seems OK.

The plastic/neoprene parts are fairly cheap in 100 qty at Mcmaster:

neoprene washers: #90133A017
rubber grommets: #9600K66
nylon 1/4 nuts: #94900A029
nylon 10-24 3/4 screws: #97263A245
nylon 10-24 nuts: #94900A011

Microstepping
I kind of knew this already, but besides vibration isolation, the other important thing that reduces vibration is microstepping. With half stepping, the noise was still too much even with the 2 modifications above. I went an extra step and made a 1/4-20 Delrin nut for X-axis because there was an audible rattle between leadscrew and leadnut; thinking that it would quiet things down. However, with half stepping, that didn't significantly help - but the noise reduction was more pronounced with microstepping. I used TinyG but I'm planning to switch over to grblShield - both are 1/8 step.

I measured the following max values using Sound Meter Lite Android app. Anything under 60 dB sounds good enough to me. Again, these are supposed to be worst case scenarios - with the disclaimer they were tested on a single axis.