Siren coming from speakers!
 

I have a 2000 S-10 Blazer LT. Last week I found the battery all but dead after not driving it for a week. I put it on a 10 amp charge overnight and it started with no issue the next morning. Since then, while the vehicle is running, the radio will cut out and I will get one of two things: dead air coming from the speakers or a very loud siren (like a european police siren or warning tones) coming from the speakers. The radio (stock) display changes from the radio station (or track number when on CD) to the time only. It acts like the onstar is engaged, but with loud tones. This will last from anywhere between 5 and 20 seconds. Then the dead air or siren will end and the normal radio or CD function returns. If I hit any onstar buttons, onstar programs immediately kick in and when cancelled, radio functionality resumes. It seems to be happening randomly, sometimes when I'm driving, sometimes when it's idling. I can go half an hour in between or 2 minutes. It's driving me nuts! Does anyone have any idea what this could be?