I like my Contax G2 (http://www.contaxg.com). It's a bit loud because of the motor. The shutter is the same assembly as that used in the Hexar RF. The classic Konica Hexar (fixed 35mm lens) is AF and the early black models had a famous SILENT "stealth mode." Expecftg to pay a premium. The lens is very good, probably on par with a 1990 35mm Summicron.
I like my Canonet too and I've pretty sure it's the quietest of all mechanical 35mm cameras (maybe the Rollei-35's compete). It's a wee bit quieter than my 6x6 TLR. Leaf shutters all.
[size=1](My old Canon digi is quieter still, annoyingly TOO quiet -- can't tell when it fires sometimes! Electronic "shutter noises" from modern cameras (including film P&S cameras) should be entirely mistrusted -- their timing is highly suspect.)[/size]
Quietest manual SLRs are probably the Olympus OM series -- Canon newer EOS's are also quite quiet for SLRs.