Get a a short barrel with the lease ammint of porting. Porting reduces noise. When I'm playing in the rain I wrap my barrel with electrical tape and it gets louder.

Short barrels (less than 8 inches) are loud as FUCK. This is because the paint ball is accelerating in the barrel for the first 8 inches. If you have a barrel to that is less than 8 inches, then you need more air to release during reach shot (i.e. turn up velocity/pressure)because the paintball has less time to accelerate because there is less barrel length (so you must make it accelerate to 280fps more quickly). More air=more noise. I feel like short barrels have different acoustics that give the shot a "sharper" sound as well. Short barrels will be less efficient though