I'm having the same exact thing happening right now. It ended up being the bracket connected from the back of the transmission pan and over the two exhaust pipes that's rattling. What happens is the bottom bracket is held by a long screw that holds both brackets together, and that long screw somehow unscrews, so you only have the top part (connected to the tranny pan). With the engine pressure at low rpm's and idling, it rattles against the exhaust..
Most likely, if your case is like mine, you'll need to buy two parts: the bottom bracket piece and the long screw.
Hope that helps!