Do you de-gas/unscrew the asa knob after each game?
If so there is air still pressurized in the gun even after you unscrew the asa knob, so it will go 'pffft' after a few minutes after you degas the gun while it is sitting on the gun stand. The gun is just purging the left over air that is inside the passages - this is normal.
Originally posted by phillysteve:
I also enjoy the "there" instead of "their" in the subject, step up your English.