Three noises a gun creates.
1. bullet breaking the sound barrier.
Answer- sub sonic bullets.
2.action of gun.
Answer- less moving parts. single shots or bolt guns.
Answer. closed action,long barrel,suppressor,less powder.
In a pistol, a contender with 14" barrel a large volume suppressor shooting the weakest charged heaviest projectile you could find.
However this is a poor shooting weapon. It would be long and bulky, inaccurate and ineffective.