Ya know, I had a noise like that coming out of the trans one time too...
i had used a bolt with too long of a head on the torque converter/flex plate. When I'd get on it, or when it would shift hard, the flex plate would, well... flex. As a result, the bolt head would hit the back of the engine block and make a gawd awful rattling sound.
Check your torque converter bolts for signs of "polishing".