I have a Gamo Bone Collector in .22; it's the quietest air gun I've ever fired.
At 950 fps (with ultra lightweight alloy pellets) it's subsonic. Uses a gas piston rather than a wire spring (smoother, quieter mechanism), but is still a barrel-cocking gun. Little over 20 ft./lbs. of energy at the muzzle; with the right pellets, good glass, and proper technique I've taken crows at up to 60 yards.
I'll measure sound next time I have my sound meter out and handy, but for reference it produces less noise than the pneumatic narrow-crown stapler I use in the garage. The 'thwack' of a pellet hitting pine is louder than the report from this rifle.
Gamo Bone Collector Bull Whisper IGT Air Rifle. Air rifles