Of the four Nikons listed in my signature, you'd need to swap the F and the F2 to get them in order from quietest to loudest.
The F4's shutter itself is very quiet. It's just the rest of the camera that makes it noticeable. To test what I just said, put an F4 on "T". Fire the shutter. Turn off the camera, then move the shutter speed dial off "T". You can barely hear the click of the shutter closing. Or, put some weak batteries into the MB-21, etc, and use the camera until it will take one shot, then lose all battery power. The mirror box is slightly quieter than the F's mirror box. I just wish that Nikon hadn't used winding motors that sound like someone sneezing.
The F is pretty quiet with a distinct click when fired. A little quieter when the mirror's locked up, but not by much. Add an F-36, and it becomes much like a motor driven F2, noise level-wise.
The F2: a bit louder than the other two cameras, but still sounds pretty nice and smooth. With a MD-1 or an early MD-2, it sounds much like a motor driven F. Pretty much identical, actually. That said, get a later MD-2 or an MD-3 with stripped gears, and you'll think that the AFLAC duck is following you around.
The F3 sounds a bit like an F2, except for a bit of a clang when the mirror box operates. Not unlike the difference between a Nikkormat FT series body and an EL (or early FM/FE series camera).
F5 is loud, except in Cs mode, when the actual shutter and mirror box combo sounds just like an F3.
FM2n, FE2, FA, and FM3a: Noisy bastards. *CLACK!* Sounds like a deer rifle going off next to your ear.
FG, FG-20, and EM: As they've aged, their mirror boxes have begun to sound like Canon A-series cameras. *squeak*
My experience with the Nikon S2 is that it sounds much like an F, with the mirror locked up. Mainly because the shutter's the same design, just with cloth curtains.
-J