Just how noisy are these little machines?
What is the source of the most noise, the router motor or the actual cutting?
Are some router motors quieter than others (Dremmel, Porter-Cable, RotoZip, other)?
Does cutting slow keep it quiet?
Can the router motor be put in a sound insulating box?
Would it be conceivable to put the whole machine into a sound insulating box?
Should I just give up the CNC idea and spend my time on something quiet like learning to play the bagpipes?
I'll try to answer your questions from my limited knowledge.
How noisy the machine is, will vary depending on the exact components and the precise construction. Having said this, though, this is what I'd expect...
Usually stepper motors are fairly quiet -- think of the noise your floppy and hard drives make...they use stepper motors.
The motor for the mill will likely be the single noisiest part. Do you do woodworking in your basement, now? How do you deal with the noise and your neighbors?
I'd probably not worry about it *too* much, unless your machine is a true "rattle-trap."
As to the precise source of the noise, my experience has been that when using a handheld router, the motor is the noisiest part. The actual cutting is not particularly noisy unless you manage to set up a vibration, somehow.
As to sound insulation, in the early days of computing they used to make soundproof enclosures for the incredibly noisy impact line printers. Basically they were an MDF box lined with the convoluted foam we now use for mattress pads. While they didn't make it silent, they muffled the noise quite efectively. I'm sure something similar could be used for this purpose -- make sure of ventilation, though, so the motor doesn't get hot.
-- Chuck Knight