wstar--I'm having the same hissing sound with my cat/exhaust set-up. I'm taking mine to a muffler shop next week to get them to check for leaks and do some seam welding. I noticed there's very little room for error in the small pipe going to the exhaust from the x-pipe. I'm almost positive that's where my leak is coming from. Nothing a quick hit from a torch won't fix!
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