If it sounds like a metal to metal slap, I go with Blade. Mine is doing the same and I finally found out that the holes in the rear radius arm brackets are egged.
My mechanic pointed it out to me when I asked him about the noise and my truck was up on his lift. You can't see it directly because of the bushings blocking the view, but what is visible is that the ends of the arms were not centered but were up toward the frame with the lift supports under the radius arms. Hope that makes sense.
Solution: new brackets, bushings and perhaps radius arms.
Good luck,
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