97 Expy. running rough idling

The truck runs fine until I'm setting at a light. Once I got it home I realized a loud hissing coming from under the hood. Sound like it's coming from the top rear area but I can't locate it exactly.Maybe a vacuum leak? Has anyone else ran into this or have any ideas?

Sounds like more than a vacuum leak. More like a vacuum hose has come completely off. Regardless, track it down and re-hook or replace that hose.

After further checking, the hose, which runs from the PCV valve, (on the passenger side valve cover) to behind the throttle body has a rubber elbow in the back. My understanding is that checking the 5.4 is by feel only, you can't see it. The rubber elbow has a history of deteriorating in time. Check that elbow. The whole hose including the elbow is about $25 at Ford. It is a hand press fit, no clamp required. 30 minute job.