DaraSue,
I'd try disconnecting the alternator and see it it stops.
Pull the alternator and get it load tested, as suggested. If it shows a short, you've probably found the problem.
200 amps is double what you need...
(See below.)
Gluaisrothaii, I noticed several D's do that when connected to a trickle charger (then turn the key off). At first, I thought it was the cheap charger introducing AC somehow, but later noticed they also acted exactly as you describe w/o it connected. I never could find exactly where the sound was coming from. They are all still working without any other issues...of course the battery will run down if you don't hear it and cycle the switch.
David, using the "wrong" bulb will not send voltage to the voltage regulator -- With the key off, the bulb has no voltage to send. If the bulb barely glows with the key off, it's usually one of the smaller three diodes connecting the regulator to the 3 stator-rectifier terminals (if used). Otherwise, at least one of the rectifier diodes are leaking.