I would say that if your 120 mm fan is too loud, it must be running pretty dang fast. A 120 drives enough air that you should't have to run it very fast which would make it pretty quiet. Have you tried lowering the speed of the fan? If you really want a 140 just get a ball bearing fan and do a little comparison shopping with the specs in mind that you want.
My radiator fan is in fact a 140mm fan, I am looking for a static pressure 140.