While driving sometimes there is an Inside bird chirping sound?

Inside as I drive the truck it makes sometimes a squeak chirping sound off and on. Goes high and low pitch. Sounds like it is behind the seat. Seats are not loose. The sound travels and it seems in the front center of dashboard. What can it be? It's driving me crazy. The sound comes and goes. When I make a stop and then forward drive forward it starts squeaking like a bird. Does not do it all the time. The truck listed above is a two door StepSide. Can you help me?