Sorry for taking so long to post again, no internet for awhile,
anyways, I had 2 factory (orignal and second) then I switched to 2 aftermarket. It's only the main 3-way cat that keeps breaking down, the pre-cat's are fine. No leak or anything, it's just that I get massive rattling at idle from the cat, like the honey-comb structure inside has failed, but it passes the emissions test.
I don't understand it. The noise happens a few months after each swap of the cat