Maybe it's not in the trans, maybe it's on the underside of the floorboards?
Maybe it's something in or on the exhaust pipes.
Doubtful, but can't say definitively not at this point.
Reviewing - these are KNOWN so far:
1. Speed is the only cause factor.
2. Noise comes on all at once, instantly - doesn't change in intensity or pitch once it starts.
3. Doesn't change with engine speed, clutch or shifter position.
4. Not speedometer cable.
5. Begins after the car has travelled a half dozen miles or so.
6. Begins at speed above 50mph.
7. Emanates from portion of transmission directly below/behind shifter.
