For as long as I can remember my truck has been making this hissing noise in the exhaust while accelerating. I never thought much of it until my buddy said it sounds like I have an exhaust leak. So I took it to a few shops and they all said there isn't a leak. The cats are fine, the manifold gaskets have been checked and are ok, and everything else seems to be good. I also can't find anyone on any forum with the same problem. Any suggestions?

You could suck some Seafoam into the intake manifold and do your own smoke test.