well if you've decatted it and it used to rattle you can't really put it back as it's blantantly fucked.

you should really notice a difference, i can only think where your cat has broken up [hence rattling] it's caused a blockage elsewhere on the system. does the car perform as it should?

ultimatly your gonna have to get a new one come mot time anyway as a mk6 won't pass emissions test without it.