hey, yes sounds like if its rattling its a cat, no worries I ran my clk430 mercedes until feathers or fluff blew out the exhaust and it stopped! lol It was anoyying for a while but saved my self big bucks. Another (my recommended) option is to straight pipe it (affordable in most cases with local muffler shop, or do it right by replacing cat. Hope this helps, good luck with your xf!