Axle nuts were tight. Going to plan B

 

i have a similar problem. When i intially start up the car, i hear this weird rattling noise, from the rear, damn it drives me crazy. you only hear it intially but i cant confirm as once i start driving, the exhaust noise overpowers the rattle. you know that water that sometimes comes out of the exhaust at intial start up, well i checked right after starting and there was a welding between the back end the pipe where it was coming out from. so i under the assumption that their is a leak in my exhaust. i am gonna take the car in this weekend and have them lift it and see if i can figure it out.

 

I had the same rattle under the same conditions as the OP. Turned out to be the rear bumper, on the bottom where it rolls under near the exhaust tips making contact with something behind it like the frame or whatnot. I found this by lightly pounding on the back bumper untill i heard it, then went from there. I took some double sided 3M tape (the gray stuff) and "padded" it between the bumper and the "frame", and problem solved!