96 f 250 OBS 5.8L 4x4
I have long tube headers and I had to replace the junk manifold gaskets that came with them originally. So I did my driver side one no problems after...finished the passenger side last night, go to start it up and it sounds like someone is sitting in my garage hitting an empty keg with an aluminum bat as hard as they can. No idea what it could be.

 

Exhaust leak.

 

This noise is LOUD and has no consistent rhythm or anything I can't really see it being an exhaust leak

 

Header hitting the frame or trans? I have had to shave the pass side frame a little to clear longtubes before.

 

You know, it could be somebody hitting an empty keg with an aluminum bat... after they drank all your beer...

Seriously, I bet your engine is moving just enough to cause the new headers to bang into something like Conanski said. There ain't much room between the right frame rail and factory manifold on a 351... I'm sure headers are no better.