Popping noise in rear wheels when I shake my truck side to side. What would cause this sound?

i have a 2003 ford f150 xlt supercrew and when i turn sometimes it sounds like something falls over in my bed of my truck.( i have nothing in the bed of my truck) so i had someone shake my truck side to side while i climbed under the back by the rear end, and found that my rear wheels (both sides) make a popping noise. What could this be?

Read more: Popping noise in rear wheels when I shake my truck side to side. What would cause this sound?