Just a small point but the whack of the pellet usually makes a louder noise than the sound of the shot and will be nearer to your targets immediate neighbours than you will be.
The quietest gun I ever shot was a daystate air wolf many years ago. It was really quiet. 6 feet away and you were not sure if the gun had discharged.... You certainly heard the impact though when lead hit cranium.
I think you will struggle to find uber quiet and super cheap tbh but good luck.