its a distinct pinging noise, not so much rattling...definitely coming from the cat...i just had a muffler shop fix a bent hangar, i asked them to check for a loose heat shield and they said it was fine
Get under there with a rubber mallet and start hitting the entire exhaust, hit the floorboards around the heat shields. Thats what i do at work and it helps a lot, trying to pinpoint a rattle. Make sure you hit the cat converter also. But don't hit to hard to bend stuff.