That sounds to me like one of the diodes in the alternator is going bad. When one of them gets an intermittant open circuit in them, it will make the voltage regulator buzz like that as it comes in/out of contact. A bad diode will also make the gen light come on and stay on. Sounds to me like you have one that works sometime, and not others. An alternator/starter shop can check your diode bridge in the alternator and tell you if something is wrong there. They are pretty cheap and easy items to replace in the alternator.