My '01 XJ 4.0 is making this odd hissing noise, which seems to be coming from the intake manifold area. It's not the constant noise of the normal air intake, but rather a sort of irregular tsst...tsst..tsst sound. It gets worse as the engine warms up, and is a lot worse at sea level than at altitude. It sounds like a vacuum issue, but I can't see any cracked hoses or anything. I'd appreciate any suggestions!

 

Exhaust leak.. Most likely from crack in the header.

 

Spray a bit of starting fluid there than you will know for sure if it's intake or exhaust.

 

Thanks guys, I'll give that a try.