I am having the same kind of problem, if you are saying that you have a rattling sound and no heat shield are loose it might be the catalytic itself that came apart inside (it is made of a stone) thats giving you that sound. Take your truck to the shop (b/c is under warranty), get the the ecm scanned (even if the check engine light is on) because the code will be stored in the computer, if is a "catalytic efficiency" code you need new catalytics, if it a "lean" code it might be a leak or a fuel delivery problem. but get that check. If they found nothing with that insist on a backpressure test. They have to do all that at no cost. By the way... there's a bulletin for the O2 sensor were they reprogram the parameters of the O2.
Hope that help, and let me know how it goes b/c mine is off warranty
