Why do i hear cracking in my legs, ankles, knees, or back when i move?

Unknown: I don't believe anyone knows for sure what exactly causes certain joints in the body to crack or pop. Theories include snapping of tendons or negative pressure in the joint fluid pulling in nitrogen gas. If it begins to be painful, i would then suggest evaluation by a physician. Simply popping the joints in the absence of pain won't cause any long term problems.