Ever see a video of an adjustment online? Crack your knuckles? Or have a joint pop?
Want to know what that is? Well this phenomenon is called a cavitation.
Cavitation gets it name from a concept called cavitation flow. In a vacuum, as a fluid expands rapidly it fills all available space. Well when there is more space than fluid volume in a vaccum something must form/fill the additional space created. In this space a vapor of the fluid forms and then collapses. This expansion, creation of gas and subsequent collapse of the pocket is the cavitation or popping sound you hear. The volume of the pop depends on a few things location of the joint, size of the joint, amount of gas, and rate of the movement.