disk swap on '74 or keep drums?

Nothing wrong with drum brakes. They probably stop better than discs.... ONCE.
Discs start shining when the brakes are applied repeatedly in a short amount of time or when the brakes get really wet. (don't ask). Lol.

The noise is probably the brakes dragging a little. It goes away when the brakes are applied because the shoes are rubbing the drum 360 degrees during the complete revolution instead of just one spot when not applied. Pretty normal noise if you feel no appreciable drag when turning by hand.