Annoying alarm when driving not seat belt what could be? 2002 ford ranger?

When I start this truck, the seat belt alarm signals- fasten the belt alarm goes off. As I drive down the road another lower volume alarm goes off for about 2 to 3minutes what might be the problem?

Could be a check gauges alarm

Are you sure your door is shut?

is there something heavy in your passenger seat that the car may think is a human and wants you to put a seatbelt on? Maybe the other door isn't fully shut