Engine idles fantastic, no loss of power.

With the hood open it's quiet up top. The sound is definitely coming from below. When I shut the truck off the hissing continues for a couple seconds, suggesting it's not a pully (second suggestion is the sound is below all the pullies. It's not coming from the front of the engine)

Ideas?


Ignore that knocking sound you hear, the truck has always done that.

 

Check for exhaust leaks probably before the cat.

 

I'll check best I can. Wouldn't an exhaust leak sound like a pop pop pop or tick tick tick more than a constant hissing? It sounds like a loud vacuum leak but there's no signs of a vacuum leak.

 

Can't really hear that good. An exhaust leak would be easiest to find. Plug your exhaust with a towel and it should get louder with more back pressure.

 

Thats not an exhaust leak, and I wouldnt suggest plugging your exhaust with a towel. My 02 4.7 makes a similar sound, not quite as loud though, and it does continue for a second or so after I shut the truck off. I've never worried about it but now that you bring this up I am a little curious, Ill look into it and let you know what I find out.

 

FWIW, my dad's 02 has always done this. Honestly, sometimes I wonder if it's the coolant de-gas bottle doing it.

 

Thanks guys, that's reassuring to hear it's common.

 

SOLVED

There's a vacuum line that runs from the engine to a box just below the drivers seat under the cab. It's gotta be some kind of emissions box, probably for the EVAP system. The connection between two hard lines was a rubber hose which had cracked. It was a massive vacuum leak.