Can you pin point where it’s coming from? Could be a nick in a hose or an o ring you could try http://www.ionowners.com/showthread.php?t=3216 about half way down there’s a bit about finding leaks
Qucik cure for most ION 'leaks'. Turn the reg all the way down - so no air is going in to the marker. Then, while firing, turn it up until you see approx 180psi on the dial. Fire a few more times at 180 psi and see if that leak mysteriously disappears (just like my sons did last Saturday ).
i recently had a leak from the long blue pipe from the regulator to the solenoid . Pulled the pipe up the hissing dissapeared, placed it back in the gun started hissing again. replaced blue pipe now alsorted