2004LSSport

i had my car idling tonight for a good half hour - 45mins, and had the defrost on the whole time. after getting back in and driving about a couple miles, i had noticed my check engine light was on. now i have been hearing an idle rattle from somewhere under my car (exhaust for sure). i was assuming it was a heat shield or my cat was rattling. i have not ran the computer yet, but could it be something else other than the possible failing cat?