1968 Plymouth GTX, mild early 70's 440, 11" CenterForce DualFriction, new input bushing, 23 spline of unknown sourcing (looks to never have been apart, shifts great though). 3.55 SG fresh, original driveshaft.
There is a noise that sounds much like an old school buzzer that eminates at speeds above 50mph and remains constant above that; it goes away consistently below that speed and finally does go away at 70+ mph.
Everything in the clutch/flywheel is new and Brewer's sourced.
Tailshaft bushing and seal are also new. Speedo gear is new as well; I had it out to inspect and it's flawless and I've eliminated the cable as a possibility as well.

The noise is not engine speed dependent; only vehicle speed makes it happen, whether clutch in/out, regardless of gear (or in neutral). Revving the engine in neutral, no difference.
It comes on all at once, like a light switch being flicked.

I have a video on YouTube of it occurring in real time:
A833 Noise - Speedometer Disconnected at Transmission

Can anyone confidently recognize this noise?
TIA!!