Horn buzzing?

[JesseW.]

Went to start the car today and the it was dead (haven't driven in a month), put it on my trickle charger and went to work in the truck

I came home and heard a low pitch but quiet buzzing sound. tracked it down to the alarm horn in the trunk, car still dead. I disconnected it and am waiting for it to charge, but wtf is going on?

 

[michael.konor]

Your car should be fully charged by now, still buzz?

 

[JesseW.]

not sure, it didn't charge (I was using a trickle charger) so I tested the battery and it had 3V..... So I have to go buy a battery sometime today, that's probably why it was buzzing.