Yes they are fairly quiet when the plastic blades are snaped on (they give two different lengths of cut) but with out can be a little noisey at times.
But having said that I've used them for horses that are terrified of being clipped to get them used to the noise / vibration on the body and they don't seem to mind so they can't be that noisy!
I would recommend keeping a small screw driver handy cos after while the blades need tightening the first few times