Voltage rising then dropping? (67 Barracuda)

Do you have stock parts? Like a mechanical voltage regulator? Single field alternator? Then that is normal to the system at idle.

You check a charging system at 1200 rpm. Raise the engine speed to 1200 rpm. How are the lights and voltage now?
Steady = Normal
Not steady = a problem. Like high resistance in the wiring or a bad component.

That noise is not right and if coming fron the alternator then go exchange it another one.

This condition of the lights going dim and bright and the fan changing speeds is normal for this system at low rpm. New cars did this and customers didn't like it, and there is no fix with stock parts. It is what made Chrysler corp change to the dual field charging system for 1970.