[tomlcollins]

Your battery has low power, which is probably causing your "diode" or whatever else it may be to hum. If you left something on then it's probably just really drained and needs to charge for a while.
Doesn't sound like an alternator simply b/c it was fine yesterday and is now simply dead. I've had cells go bad in my battery before literally overnight, to the point where the car was undriveable the next day. Just take it to advance or autozone and let them test it for you.