If you mean the exhaust flexible joint part then your probably find its split the flexible part below the woven outer part. You will also notice exhaust gasses coming out said part.
You can get a replacement flex part and take the header off the car to an exhaust specialist to get it replaced.
Or the other thing it can be is especially common on Euro4 cars, the cats come loose and rattle. Had one where 3 of the 4 cats had come loose.
