I think one of my cats is rattling too. I dont know if its cost effective but I think I'm going to go for HJS cats again (there are cheaper cats out there). I had them on my old S8 and they were superb.
http://www.hjs-tuning.de/de/3/71/0/s...universal.html
This place sells the ones I want for a ok price. I'll need two of these HJS-Nr. 90 95 0161
You could probably get away with 1 or two smaller ones.
https://www.turboperformance-shop.de...s-bis-3500-ccm