Ever since I did my cat back and now even more so with the headers (MAC midlengths) whenever i go over bumps in the road, right under the passenger seat or maybe a little in front of it i hear a rattling noise. Almost like the exhaust is bouncing a little bit and hitting the underside of the car.

I got underneath and tightened everything up a little bit, but still no use. I also was pushing on it too to see if it has much flex or movement when using force on it, and it seems pretty sturdy and solid.

Any other ideas what it could be???