Possible plenum leak?
So I just installed my 5/16th Motordyne plenum spacer and the install went great. Except, while at idle, the car makes a somewhat quiet hissing sound (enough to notice) at the lower left hand side of the plenum and when I turn it off, it hisses very loudly then disappears. Before this, I took it for about a 15-20 mile drive with the new spacer and it drove just fine. I don't have any check engine lights and the idle is right around 700-750 while warm. When I started it for the first time (cold, of course) it idled at around 1000 then dropped as it heated up. So to sum this all up, the car idles fine, I don't have any check engine lights, and the new power seems to be there. What could be making the hissing sound?
Thanks in advance.