TWJ,
L.J. is correct, it is either the CDI ignition or the alternator. It could also be the fresh air fan or A/C fans if on. Testing for that is easy – simply turn them off. If the ‘hum’ frequency varies with rpm, then it is most likely the alternator or high voltage ignition.
There is supposed to be a radio noise suppression device plugged into your voltage regulator. Confirm that it is in place and in the circuit between the engine connector and the regulator.
Do you have solid core ignition wires? Are the plug connectors radio suppression resistor type? There may be six radio suppression resistors plugged into the cap terminals.
Is there the small ground strap from the back of the alternator body to the engine crankcase top perimeter stud & nut?
Is there the braided ground strap from the electrical panel to the chassis ground behind the MFI fuel filter console?
There are aftermarket radio noise devices that can go in the power lead to the radio. Do not use a long power wire with the excess coiled up.
You might put a small (and short as possible) braided ground strap from the radio chassis to the car chassis.
You might add a small braided ground strap from the left chain cover to the electrical panel at the voltage regulator.
It is also possible that there is a defective electrolytic capacitor in the vintage radio power circuit. After all it is now 35+ years old.
Best,
Grady
(EE Carnegie Mellon ’68)