Car wont start...pulsing sound in the speakers?

[beegeee]

First off my battery is a year and a half old. My car has died on me a couple times. I'd be driving through the middle of an intersection and all of a sudden its dead. I've always been able to re-start it until today. This morning tried starting it and it wouldn't turn over. The lights on the dash worked but it wouldnt turn over. It was, and still is, making a strange pulsing sound through the speakers. Anyway, it did finally start. I'm not sure why...just all of the sudden it started like nothing was ever wrong. Now tonight its the same deal....except it wont start for me...and its still making that pulsing sound! Help!! Its a 2001 Pontiac Grand AM GT.