[AZKen]

Humming/grinding noise does not do it for me as a description. It's very hard to tell which of the noises I hear is the one you are asking about. I hear a low pitch rumble and a higher pitch "whine". First sounds like exhaust. Second sounds like normal gear mesh noise. Sounds like many old trucks I've had. But noise from a video/cell phone is very difficult.

All I can say is experiment with the clutch pedal. Push clutch pedal in when the noise starts and see if it goes away. What noises are there when down shifting. Do some coasting with clutch pedal in to see what noises you hear at different coast speeds. Try to run on different era paved roads. Do you have cruise/car style tread tires? or snow/mud..... or radical tread design? Bias, radial....

Do you feel any vibration frequency that matches the sound frequency/pitch/tone?
What viscosity lube in trans? Synthetic lube. "GL" number?

What outside temp do you have now?