solenoid valve

In my '97 it sits atop the intake plenum. I can hear a loud hissing sound here. I think it may be one of the vacuum lines leading to it. Out of curiosity, I took the green cap off the EVAP port to see how much vacuum there was, and I felt none. Is that the way it's supposed to be? I'm trying to figure out if the leak I hear is from a vacuum line or an IMG. Any insight is appreciated. By the way, the truck runs fine. Let me add that the valve does a lot of clicking...