hey guys, i think ive confirmed what the rattle is. ive fixed my last week.

went to put in a high flow cat and found that the catback pipe (where the flange connecting to the cat is) was too close to the chassis. to give you an idea how close it was, you couldnt put your fingers in between the space. basically got the hi flo cat customised so that the pipe sat a bit further from the chassis and that changed everything. its got something to do with the water hitting the hot pipes making it expand and vibrate.

this also explains why getting new headers changes it because it also moves the pipe further away from the chassis.

anyways, hope this helps!