Does it leak whenever air is in the gun, or only when you fire? Sounds like an O-Ring could be gone - Take the bolt out, check orings (changing any dodgy ones) and make sure it is well lubed.
Could be a hose attached to the noid also, but try the bolt orings first if you suspect it is coming from there. Can you feel the air leak when you put your finger into the breech?
-Will