Check the tailpipe bro...
I had the same rattle coming from my back end and it was due to the rubber tailpipe hook being in the wrong place allowing the pipe to bang against the bumper. Took it to the shop and they "tried" to fix it but it came back like 2 days later. So I fixed it by installing the comptech exhaust. Anyway.. check it out, Im betting thats where the noise is coming from.

Congrats on the new ride.