This particular one is very nice and smooth - was serviced a couple of years ago, and it doesn't have the rather loud "clack" when triggered that some older, "unserviced" S2's can have.
But an S2 is never as silent as an M. It has a special brake in its mechanism that makes the typical sound. I found this description on the web:
"With the increase in speed of shutter curtain came larger rebounds when the shutter stopped. As a result, the image became 'chipped' and belt-shaped reexposuring was prone to occur. This is where the idea to employ a pendulum-type brake was conceived. When the shutter curtain is drawn, a 'weight', which has the same mass as the curtain, hits and stops the curtain.
You might have heard of this mechanism someplace else..... that's right, this mechanism is used in one portion of the Nikon F5's mirror-bounce protection. It is referred to as a 'mirror balancer' in a Nikon F5 catalogue, as well as in articles that appeared in a technical guide. This terminology, however, is not altogether accurate. A more correct term would be 'mass brake used for a mirror'. The word 'balancer' comes from the idea of a brake spindle which was designed with emphasis placed on weight balance, so that the brake would work exactly the same no matter the direction or position of the camera. This innovation, of course, found its way into the F5 following its success in the Nikon S2.
The 'flick' sound made when the shutter of S2 is released is made by this mechanism."
Very interesting, but I've never seen an illustration of how it works.
Erik.