If you have the Magnaflow exhaust with the "rattling" sound that won't go away, there may be an answer. After ensuring that no part of the exhaust was in contact with the car or heat shields I was left scratching my head as to where the rattles came from....It turns out that the rattles are coming from inside the first resonator. There is apparently a bad weld on the baffle or something inside. Magnaflow is replacing mine and has said they "may have a bad production lot" of them. This after several reports of rattles.
Next time you are up on a lift, tap your resonator with a rubber mallet. If the sound resembles a snare drum , you may have found the problem.

 

I had the rattleing too but it was just a hanger that had contact with the heat shield