[AZKen]

Do you mean the splines of the slip yoke or the Ujoint of the slip yoke?

Put your hand on the trans shift tower, where stick come out and see if you feel the noise. Stick shake.
You are an engineer, you know about natural frequency. This can be similar. You mention it starts at a certain RPM and stops and another. This makes me think of amplitude, resonance. High frequency "rattle" sounding like gear or bearing problem, but is not.

The 2, 3 and 4th gears are helical and don't make much noise. But they go make some noise. Noise at 1:1 is said to either be normal in SM420 or bearings. Additives may quiet it. Lucas is said to be best.
Old trucks with all metal cabs and no sound deadener, have always made noise. They are a huge transmitter.
Make sure ujoints are phased.