Originally Posted by Zeto

Ok guys under much research and testing it is from the test pipes and possibly HFC. I bought 2 sets of test pipes, the FI resonated ones and Berk test pipes. They both created the hissing sound. The FI resonated ones a little less but it was still evident. I went back to stock cats and the beautiful exhaust sound was worth the sacrifice in power. With that being said I'm selling 2 sets of test pipes, any interest? lol