I heard that Solar & Palau makes some quiet fans. I personally own a 4" and a 6" Valueline and they are not quiet, but I have a duct noise reducer and fan speed controller which both help reduce noise.
Good idea, that never even crossed my mind, now just to find a fan with a quiet motor... brought one from argos today (uk store) but its too big and makes a hell of a noise even though it was advertised as quiet operation, thats going back tomorrow.