If noise is the main concern you might consider a pellet gun.
http://www.airgunsbbguns.com/Pellet_Guns_Break_barrel_Adult_s/342.htm They have about the same knock down power as .22 cal. I prefer the 22 lr mainly because of ammo cost it's cheap compared to the other choices like 17 or 22 mag. If you are not a target shooter and buying it to use for occasional varmints only then I would go with the 17, or 22 mag. I use to own a 22 mag and have only shot a 17 once but I really like the accuracy of the 17. The little bullets look like miniature 30-06 shells way cool.
