I don't know for sure, but I believe MF Part #14616 was used as the resonator. The x-pipe was custom made by the muffler shop. The MF mufflers are 5"x8" oval. The entire system is fully welded.
I just went through the frustration of having this same rattle you guys had. Turns out it was the resonator. I had my trusted muffler guy replace it with an x-pipe, two in-line Magnaflow resonators, and Magnaflow mufflers. I had been tinkering with various exhaust setups and am quite pleased...