A hissing sound back by the EGR? I'd say check for leaks then. I don't believe there should be any hissing sounds at all...

 

Ya, the only thing i could figure out was if somehow the diaphram in the egr valve had been puntured some how. I vacuum tested it after I cleaned it and it checked out ok and that is the only thing that I can think of right by the firewall that could hiss.......unless one of the Egr bolts is lose....

 

Did you use a vacuum pump on it or only teh pinch tests?