Depends on the critters.
You can feed a sub sonic round into a weapon you currently own and most civilians wont have a clue if you fire 1 shot.
It is when there are multiple shots people realize what is going on and start to phone the police.
Caliber is also going to matter, not only for which kind of critter you are shooting, but also the amount of noise reduction you can expect. There are numerous 223 cans out there for AR-15's & threaded rifles, combined with subsonic ammo, makes it very hard for people to identify, especially at range.
If all else fails, get closer, and use a bow. Critters like food, and if it is a dumb critter, i.e. a type of "pig," then just feed them and shoot them.