Sounds to me like the infamous "can of marbles" sound that came from the torque converters from the earlier 4R100 tranny's. When the TC was unlocked, like when parked at idle, it would sound like a "coffee can full of marbles". There was a test for the condition, something about locking up the TC at parked/idle by grounding the lock-up signal wire or something like that. If the sound went away when the TC was locked up, then you had a "coffee can" TC. Hopefully someone who knows more details of the test will chime in later.
Way back when, before I got my BTS, I had a "coffee can" TC. While somewhat annoying, it ran for a long, long time before the tranny itself crapped out and had to be replaced.