Assuming it's hissing while it's not cocked it could be...
1) spring tension. a weak valve spring combined with a strong main spring (or dialed in all the way adjustable bolt).
2) bad cup seal inside the valve.
or if it hisses when you pump the pump back for a brief second, then you're low on air. Not enough air pressure to hold the power tube shut until the sear catches the hammer.
But I'm no expert. Let's see what others have to say.
12gm bucket changer
Acryllic tube: ~1" o.d., i.d. paintball sized, 15 balls in length