Loud buzzing moaning sound with headlites

On my 87 300TD, when I turn the headlights on whether or not the car is running, there is a loud whistling, whirring sound under the hood. If I turn the lights off, it stops. When I turn them back on, the sound comes back. Everything works fine. Is it the voltage regulator. Neighbors can probably hear it when the engine isn't running! Any ideas? Thanks.