Could be the bushings, im not to great with transmissions. My first thing would be to check for any loose bolts or things along those lines, the lower RPM's and higher load on the engine could be vibrating just right to make something rattle. Might even be an exausht shield over the cat to stop leaves from getting trapped inside, that happened on my 89 Mazda 929. For the record, its a waste of metal, so if it is loose just rip it out you dont need it.