I'd guess this is the original converter, which were a ceramic core type. Since the early days, the rattling V6 catalyst (and the 1.8) have been an issue. I remember when lots of V6 owners were complaining about the EML warning accompanied by a rattle from the pipe. The secondary catalyst tended to detach from the internal fitment. There were several proposals on how this could be fixed, including a hole drilled at a specific point and a spot weld applied.You might attempt a historical search using appropriate keywords to find the thread, which will be over a decade old I'd guess. Realistically, I suspect replacement is the likely answer. It was for me.
TC