I have that same issue too.
This started happening after I had my catless down pipes and AE exhaust installed. My buddies and I put the car on a lift to listen and we think it might be my waste gates? It's noticeable when I'm accelerating or when the car is decelerating. It's really annoying and makes my car sound like a civic with a ghetto chopped exhaust. Haha.
I remember there was a remedy for the rattle by adjusting a value for the waste gate or something on the Procede, not sure if it's the same for JB4. Hopefully someone will be able to chime in.
