I would recommend a Co2 gun, pistol or rifle with a moderator.
Springers will be loud, not due to muzzle blast but from the spring inside the action, it really is loud. Pumpers can also be loud so I'd also stay away from those. All of my Co2 guns have a moderator and are a lot more quite and neighbors don't know when I'm shooting. All can be heard is the hammer hitting the valve and a blast of Co2 from the barrel. I would highly recommend looking at the Crosman forum, use the search and you'll find plenty of info on how to build and customize the gun, or, look at the classifieds, lots of guys sell their guns that are already modified and quite, and most of these guys tune for power so it's not like you're buying a stock gun. these guns put out A LOT of power besides being quite.

http://www.network54.com/Forum/275684/

This is one that I built from after market parts, very quite and has power enough to take a pidgeon at 40 yards.
[Linked Image]
This one is my long range sniper rifle.
[Linked Image] 1/4 inch to 3/4 inch 5 shot groups at 55 yards are normal for this one, but normal shots are taken beyond 70 yards. This one is even more quite than the first gun pictured and will shoot a 14.3 gr pellet at about 12 ft lbs, that's more than enough power to take a rabbit at 40 yards with the right shot placement.
Good luck in your search

S.W.A.G. it