Last Thursday, when I got in my car and turned the key nothing happened. No lights, no horn, nothing. I took the battery out and replaced it with a new one and the car started right up. On Saturday I took the car to Auto Zone and they tested my alternator and said its fine. Well earlier today I get in the car and turned the key, the car won't start again. This time when I turn the key, I heard a low humming noise that seems like it's coming from under the car and that's it. The lights come on and the horn is strong. Does anyone know what it may be?