I was surprised to find just how quiet mine is. At least without any tools on it 😀
The X/Y/Z motion is actually considerably quieter than the same motion on my 3D printer. The linear bearings on the 3D printer are just a lot noiser than the bearing setup used on the MPCNC.
If you’re going to use it for 3D printing or with a laser or pen then it’s no big deal and super quiet.
Basically the only noise to worry about is the noise of the tools you mount to it. If you can get away with using a router in your apartment without upsetting anyone then the MPCNC with a router on it should be no different. The only real difference I’ve noticed is that the MPCNC is more patient than me so it will run tools sustained for longer periods of time than I do when working manually.