Determine the diameter of the air tube.
Acquire a piece of clear PVC tubing slightly larger in diameter. You will only need a piece approximately 1” long?
De-gas the air tube.
Cut a slot lengthwise in the tubing.
Check the fit of the PVC tubing in the air tube.
If necessary, remove material of the PVC tubing lengthwise until the tubing fits snugly but doesn’t overlap.
push the PVC tubing into the air tube about six inches.
re-install the air tube.
This will dampen the resonating frequency created in the air tube from firing the gun.