A couple of thoughts:
1) Pyramid Air is the easy button, you can sort guns by loudness and price. They will also test fire YOUR gun for $10 extra, called Ten for Ten, so you get a good one.
2) A rifle will normally be quieter than a pistol and you'll (arguably) be more accurate with a long gun on critters. It's best for the vole or whatever to kill it w/the 1st shot.
3) Quiet costs more than loud. Don't blink or complain, that's the deal.
4) Larger caliber is louder than smaller caliber so .177 may be for you. Stay with pellets not bb's though. BB's shoot through a smooth (not rifled) bore - not accurate.
5) A springer makes a noise upon trigger pull but it's kind of mechanical, not as "gun sounding" to neighbors; it's not as alarming as a pump, pcp, etc.
6) Personally I'd not fit a quieting device to any kind of gun, even a non-firearm. Pay a little extra for a factory made quiet air gun. Stay clear of expensive legal bills later, if the local law enforcers take issue with you modifying a non-quiet "gun" yourself without the (in their view) correct legal paperwork. Better safe than sorry.