Calling all SuperCrew Owners!

I have an '03 SuperCrew w/~58K miles that has recently developed a light "popping" noise which seems to be coming from both the right rear and left rear corner of the cab at intermittent times. The pop can occur at any time, under any body/frame flexing position/condition you can think of. It even occurs when traveling down a perfectly flat, smooth road. I cant seem to find any one thing that creates the sound

Has anyone else had this problem? If so, what was the fix?

Thanks!