1999 Ford F-150 Beeping

I have a 1999 Ford F150 V8. I was told at the Ford place it did not have a keyless entry, but the theft light would come on. I took out one radio and put another in and a beeping started so I tried both radios again and the same problem with both. The noise sounds like a door chime but will continue beeping for about 3 mins after crinking. What could I do to stop the beeping. Wires? Chips? Anything.