[Here come da goat]

I’ve a 1970 GTO, 400 non RA, bone stock.
Recently replaced battery, alternator and voltage regulator.
No perceived issue charging, but there is a hum (squeak) from VR at idle and low RPM. At roughly 1,100 RPM it goes away.
When at idle (and while the hum is there) I am getting ~13.8 volts out of system, at RPM above 1,100 the hum goes away but voltage then drops to ~12.5 volts.
The new alternator is a 65amp from Ames, AC Delco VR. I do not have A/C, and this happens with or without lights, stereo or heater blower running.
I think the stock alternator was 58 amp, (not sure) and it has since been returned for the core charge.
Gen light comes on when hum is there, gen light dims gradually as RPMs climb and then goes out at 1,100RPM. This is full time issue and not limited to cold or warm engine.
Seems very odd I would have more volts at idle than at higher RPM.
Please any advice, greatly appreciated!