Which means the car was probably modded and changed back to oem before it was sold to you. Like I said, some people have put the stock exhaust back on and it still happens. Some people have changed the headers and it still happens. No one really knows what it is and a lot of mechanics havnt been able to work it out either.

I've got the same problem. Gonna put in a high flow cat in and maybe weld the cat to the cat back. Let u know if it works lol