1987 Ford F-150 Beeping noise

I got a 1999 ford f150. I had an after market cd player in it, my brother gave me another one and I decided I would try it out in my truck, after hooking it up and turning the truck on the door chime would beep, then when it stopped another beep that was different started beeping. It will do a tune of beeps 2 times in a row before stopping. Sometimes the beeps will start back up when I come to a stop. I put my other cd player back in it that was in there before and it still does it so its not the cd player. What could the problem be that make the beeps start? It sounds like the beeping is coming for under the steering wheel area.