I have an FM-2 that I purchased in 1980. This is an extremely rugged, olf-fashioned mechanical camera. My sense is that the FM-3 is simply the next generation in the design. I also have an FE-2 - same camera but with automatic exposure capability.
This camera is not silent. I have a motor drive on my FM-2, and the noize of the motor completely masks the shutter noise. With the FE-2 there is a distinct "clunk" - not as quiet as, for example, some of the 1980's vintage Olympus models, but also not nearly as noisy as the old Mamiya-Sekor 1000DTL that I replaced when I purchased the FM-2.
SLRs inevitably will make noise - the mechanical actions involved in flipping the mirror and moving the shutter curtains generate sound. If you want quiet, you have to go with a leaf shutter - and that essentially means a rangefinder camera. Of course, the tradeoff is that rangefinders don't provide the lens exchange flexibility of SLRs (and certainly not at the same price points.)