Keeblerz28

It doesn't happen all the time, but sometimes when I am backing out of a parking spot I get a rather loud whine. It kinda reverberates throughout the cab/bed so its hard to tell exactly where it comes from. Only happens in reverse (that I can tell), so I'm not going very fast, and the truck is in motion. It seems to make the noise for a few seconds, be gone a few seconds, and then back for a few sec, etc. The only things I think that would do this is either the axle or rear brakes. Axle has never leaked, so I've not changed the fluid, or added anything. Truck has 46k, and I really need to rip the brakes off (this summer maybe) and change em out to ceramics or the like so my wheels don't constantly look like sh!+. Anybody have this noise? What did you do that made it go away? Thanks for the help
Truck details are below