Never had this happen before.
While comeing home last night there was any mighty bang from under the car, and the exhaust started blowing. On insection the rear box had exploded along the seam, and the 2 centre boxes have deformed slightly.
I think the wadding inside the box must have some how blocked the tail pipe and allowed presure to build up in the system, untill it gave way, but car was running fine and didn't start missing or any thing.
Anyway, would have needed a new exhaust soon so not a big problem, Just a new one on me.