I have a squeak that I can't track down too. Mine is more of a "whoop whoop whoop" noise. The noise is identical to the noise you hear when a pigeon takes off. It only does it when the truck is warmed up and moving at slow speed but sometimes it will continue to do it for a couple seconds after the truck has stopped.

My solution to the issue is a good sound system in the truck.

 

i had the same issue for a while. the top of the gas tank rubs on the pinch weld at the bottom of the cab between the bed and cab. wasn't enough to wear a hole or damage it but just enough to make a plastic on metal squeek. try spraying some wd40 down there.