Beeping noise in 1999 Explorer

Every time I start my 1999 Ford Explorer It makes a loud beeping noise similar to seatbelt warning sound.Every 30 minutes or so it does it again as long as the vehicle stays running(even with all seatbelts fastened even if no one is sitting in the seat.Any ideas as this is driving me crazy.

My guess is a bad Sensor…door ajar,fasten seatbelt. Buy a Manual