I haven't figured out exactly what makes it or what conditions but there seems to be a chirp coming from the rear end. I have heard it most when driving slow in 3rd gear or coming to a stop when there isn't power being applied but not using the breaks either. I'm praying it's just a bushing that needs some juice but the paranoid side of me things it may have something to do with my rear locker. Then again my X has become quite the rattletrap lately. These roads in the high desert really blow.