My seatbelt chime goes off even when i am buckled?

I have a 2003 Ford Explorer Sport Trac. When I get in and start my truck, and buckle the seatbelt, the seatbelt chime still goes off. So if i cycle the ignition off and then back on a few times eventually it stops. What are some possible problems that could result in my truck thinking i am not buckled?? Thanks