I am experiencing a problem where I hear a sound while driving in the range of 50-60MPH on relativily flat or small uphill grade the sound does not occur (that I can hear) outside of that range.

Specifics
a.> If I let off the gas the sound immediately disappears, but will reappear if I press on the gas again.
b.> If the speed is less than 50MPH I cannot hear it, throttle position does not seem to matter.
c.> If the speed is over 62ish MPH I cannot hear it, throttle position does not seem to matter.
d.> It seems to occur on relatively flat or small uphill, very little throttle to maintain speed.
e.> no vibration in wheel associated with the sound.
f.> overdrive does not seem to matter

I describe the sound as: hollow metallic grating sound like when you drag the end of a chain link fence post over concrete. high pitch
My wife describes the sound as: metallic hum or ringing medium to high pitch

I have replaced the front u-joint as a start, but it did not help the issue.

Vehicle is a motorhome (Fleetwood Tioga) with 42K miles on it