Ok so stock cats made this thing SO MUCH MORE quiet. I don't think it got rid of the hiss, it just made it much more quiet... I still believe there's something wrong, maybe it is the exhaust though. I'll try to make a sound clip. I tried to shake resonators, and doesn't seem like anything inside of it is loose or broken, but then again, I can't shake it with the same force that the fumes do when revving the engine lol.

 

im willing to bet it was the test pipes mixed with the hks unique sound. my buddy has an hks with test pipes and from your description his sounds the same. my car with the single has a wierd hiss also. i have test pipes m2 y pipe and the megan single. i thought about taking the heat shields off above my test pipes and see if maybe those were vibrating

 

Yeah, I put stock cats on and it made it much better, but still when dropping RPM, or idling, when you stand behind the car you can hear the hissssss... or when you turn the car off and it completely drops in RMP thats when u hear weird *** sound. :/ blah I'll make a vid clip lol.

 

Ok, so I put the cats on, and basically what happens, when you let it idle for like 10 mins so the engine warms up and drops in idle RPM,

It sounds like a hiss, or w/e you wana call it, as if there' no engine in the motor and its just metal rubbing against metal, and you can hear it in the rear from the exhaust lol... I'm clueless to what to do, or which part of exhaust could be failing. I will make a video and post it today....

Cats seemed to make the overall exhaust just more quiet, but didn't get rid of that noise, so obviously something wrong with the exhaust... Just not sure which part, resonators? mufflers? Any tips would be appreciated once you hear the sound clip...

 

I have the same issue, well kinda. Sounds like a ball bearing is in my exhaust in the resonators bouncing around. Only hear it at idle and when rev down or letting off the gas in gear.