Same problem with the same damn headers. David Weaver at ART is putting on some different ones for me. You might check on what he is using. Mine sounded like a lawn mower with the choke on though...a pretty major leak. Looking at your cars mods, we are basicly have the same setup. I am running stage 3 heads and cam in addition to the headers/corsa/random cats. My worry was that since I am on the lowest setting in the front on the Motons that I might have caught the header on something and caused the header to pull away a little, or maybe the gaskets just ****. Let us know what gets yours fixed, and I will do the same.