Noises problem
1999 Audi A4 4 cyl Two Wheel Drive Manual 92000 miles

Hi, have a very annoying tinny rattle coming from my exhaust, mainly from the cat. Have replaced the entire system (accept manifold) and the exact noise remains. Took the car to Audi and there recommendation was to change the cat (of which I have tried!). So, I tested this by temporarily using my own constructed de-cat pipe, the noise then was herd from the center box. i have heard that fuel/air mixture can cause this problem, so tested by removing the MAF to no avail. I am pulling my hair out and any suggestions would be received with gratitude! Thanks