I dont know why I didnt think of it earlier. Someone else did. But now that I think about it this does sound like a converter clog. Drill 2 small holes in the pipe before the cat, if that fixes it then that is your problem. My friends MX6 did the same thing and that is how we checked it. If it makes a loud noise that sounds like air rushing when you crank it after you drill the 2 holes then it is a bad cat. Its not hard to have 2 tiny holes welded up.