Beeps on 1999 Mercury Mountaineer

When you start the vehicle and shut the doors a loud series of beeps occur, pause and reoccur, this continues for 2-3 minutes? What could be the problem?

Not familiar with Mercury Mountaineers but could it be a seat belt warning device? Perhaps call the Mercury dealer there service department should be able to tell you over the phone what this noise is.
1 more answer
It could be your door switch is not working and the car thinks that the door is open, or your seat belt is not buckled or that its switch is not working correctly. Any RED icons or symbols on the dash when this happens?