i had an exhaust leak which sounded like it was coming from the manifold. i had the manifolds redone with new manis and had everything milled. i STILL have a leak somewhere, albeit not as loud. my air rail is also new, so thats not leaking. where else could i get a "pff pff pff" ticking kind of leak? its driving me up a wall.

 

I'd check the Y-pipe right where it meets the cat.

 

Check the donut gasket coming off the manifold, those get old and make the 300 sound like a riding mower

 

I don't think the EFI manifolds use a donut gasket, but yes, I remember when it went out on my stock carb'd manifold and it was pretty obnoxious.

If you are pulling your hair out and have checked everything over, it might be time to just take it down to an exhaust shop. I know a lot of them around here don't charge for that kinda service just to check and see where a leak is coming from.

Other than that, maybe one of the AIR rail ports didn't tighten down all the way? The O2 sensor port is loose? Have you checked the EGR pipe that comes out of the rear EFI manifold? The 1-way valve for the AIR pump can also rot/rust out and cause a bit of noise.

 

the only thing i can think of is that the air rail isnt snugged up enough. it just sucks taking the top intake plenum off to even try to diagnose that (which i doubt is leaking). ive had this truck at tons of exhaust shops and nobody seems to know anything. egr pipe is new and so is the air pump pipe. i only have 1 o2 sensor, correct? because i just replaced one recently.

 

il crawl under the truck when it stops raining