2001 Dodge Ram starts fine then after a few minutes idles rough and makes a hissing noise.

Yep its from the engine but it is random. Some times its there sometimes its not. Wouldnt a vacuum leak be there all the time?

Just checked it out and found no leaks. The hissing noise is coming directly from the intake on the throttle body. It starts out light then gets loud and the truck idles rough to the point where it almost dies. If I push the gas it goes away for a brief moment but then slowly comes back. If I put my hand over the intake while it is happening and keep it there until the truck almost dies then quickly remove it then it stops hissing for awhile again. Wait 5 minutes or so and it comes back. In short, it sounds like it is trying to get more air.

I have a 2001 dodge ram 2500 5.9 that does the same thing! In the morning I start it up and after idling for a min. Or two it starts to idle rough. If I try to drive it it looses power and backfires and will dye if I stay on the gas but if I let off the gas it continues to idle rough. I found if I shift into nuetral and rev it up it clears up usualy or if I shut the engine off and restart it it clears up. When its doing this I can hear a hissing sound coming from the throttle body. I thought it was the IAC motor but I replaced it and it still does it. Ive changed plugs, wires, cap, rotar, intake gasket, cleaned throttle body. There are NO engine codes! This problem always happens right after starting the truck after its ben sitting and the engine has cooled off. When the problem occurs now I just shut the truck off and restart it and it goes away instantly. I'm very mechanicaly inclined and have tools and garage to fix anythig as I used to race cars and have done just about everything from building engines to changing tires. Ive worked on everything from FreightLiners to snowmobiles and lawnmowers. I currently work at a John Deere dealership and I cant for the life of me figure out what the hell is causing this! I do have a few more things to try though and I will let u know when I figure it out!

I swapped out the IAC and the hissing has stopped however, now I get a code that says the IAC is bad? All other problems have remained the same.

Anybody ever swap out the MAP sensor for this type of problem? Any chance that it could be the problem?