My '99 150 (4.2L V6) seems to be having a bit of a problem...

When I'm on the throttle, everything is fine. It's when it's idling or I'm not on the gas that it starts hissing. Almost like the sound of boiling water or an air compressor. I've checked every hose I could think of, and I found nothing. It could be nothing, but then again, it may be more than I think. Ya never know.

Anyone ever had this problem or have any ideas of what to check?