Why is there a loud buzzing alarm when I turn off ignition

What is your question?

Why is there a loud buzzing noise like my headlight reminder going off when I turn my key off it starts out quiet then gets really loud and won't shut off and my battery gauge drops below 18 instantly when the key is off and starts making the sound. It just started randomly no lights or anything left on when shutting off.

What makes this problem better or worse?

I disconnect the battery

How long has your 1993 Chevrolet G20 had this problem?

Just started today