It really depends on what you are using for a spindle, bit and what type of material that you are using.  Statistically larger diameter bits will produce more noise.  The same is true for using a router as a spindle instead of a CNC spindle like the HF500 from STEPCRAFT.  It also depends on what you are using for attachments.  For example, if you are using the 3D Printer, foam cutter, drag knife, engraving point or wood burning pen, then the only noise you will hear is the rhythmic whine of the stepper motors working in unison together.  If you are using a Dewalt DW611 router with a 1/4" end mill then the noise will be much greater; about the sound of an electric hand mixer but not as loud as a blender.
If noise is a concern for you, then you can consider getting a STEPCRAFT Enclosure which will reduce noice by as much as 50%.