I'm a little confused here.
How would:
(1) a semi-auto that's rigged to require manual recocking/reloading for each shot,
or:
(2) a pump rifle that requires manual recocking/reloading,
be any quieter than a bolt action rifle that also requires manual recocking/reloading for each shot???
Also, to answer your initial question, years ago I had a Winchester Model 67 single shot .22 rifle that had a 27" barrel and was incredibly quiet with standard velocity long rifles and almost completely silent with shorts. When a short was fired, it sounded about like a small twig being snapped.