1999 Mustang GT Beeping noise.

Has anybody ever come across a beeping noise that sounds like it's coming from the door, or left side of the lower dash area? Took my Pony out for a ride today and as I started to pull out of the driveway it started this strange beeping noise. Kind of like beep beep_ beep then a few seconds after another beep. This would repeat every couple of minutes but it did seem the longer I drove the slower it appeared. I'm not talking about the dinging chime sound that comes from the seat belt alarm, door open or the key left in the ignition. Any ideas.