I have recently read about this and what you are hearing is air bubbles popping between the joints and does not cause any harm like once they thought it did.

What you are hearing or feeling is something like the 'gas' and fluid between your joints. Or when they move the air or fluid is going from one tiny space to an empty space. Sometimes I can move my neck and crack it and I hear like bubbles or something. I asked my chiropractor what that sound was and he said it's the little gas bubbles moving around and they get amplified in your head. It's perfectly okay for it to happen. Just the gas, fluid, cartilage moving around:-)

My guess..Air pockets in the cartilage?

Cracking joints From Wikipedia, the free encyclopedia Jump to: navigation, search Cracking joints is the practice of manipulating one's joints such that they produce a sharp sound, likened to cracking (also likened to popping, etc.). The most common form of this occurs during knuckle cracking, a process different from snapping one's fingers. It is possible to crack other joints, such as vertebrae.[1] This sound is often unpleasant to nearby people, while benign to the one producing it.[citation needed] Contents [hide] * 1 Causes * 2 Source of sound * 3 Repercussions * 4 References [edit] Causes See also: Joint manipulation To produce the clicking sounds, many people bend their fingers into unusual positions. These positions are usually ones that their own muscles are unable to achieve, and which are not commonly experienced in everyday use. For example, bending a finger backwards away from the palm (into extension), pulling them away from the hand (distraction), compressing a finger knuckle toward the palm (into flexion), or twisting a finger about the first bone's axis (torsion). Cracking within the body may also be caused by a breaking bone. The snapping of tendons or scar tissue over a prominence (as in snapping hip syndrome) can also generate a loud snapping or popping sound.[2] [edit] Source of sound The physical mechanism is as yet unproven, but suggested theories include: 1. Cavitation within the joint—small cavities of partial vacuum form in the fluid then rapidly collapse, producing a sharp sound. This explains the popping that can occur in any joint, such as during spinal manipulation. Synovial fluid cavitation is the most likely theory and substantial evidence exists in support of it. (Discussed in detail below.) [3] 2. Rapid stretching of ligaments. 3. Adhesions being broken, which simply means that as two cartilage surfaces are pressed together, they form adhesions, and when the joints are separated this makes the popping or cracking sound. Of these theories perhaps the most popular is cavitation. When a manipulation is performed, the applied force separates the articular surfaces of a fully encapsulated synovial joint, which in turn creates a reduction in pressure within the joint cavity. In this low pressure environment, some of the gases that are dissolved in the synovial fluid (which are naturally found in all bodily fluids) leave the solution creating a bubble or cavity, which rapidly collapses upon itself, resulting in a "clicking" sound. This process is known as cavitation. The contents of the resultant gas bubble are thought to be mainly carbon dioxide.[4] The effects of this process will remain for a period of time known as the "refractory period", which can range from a few minutes to some hours while it is slowly reabsorbed back into the synovial fluid. There is some evidence that ligament laxity may be associated with an increased tendency to cavitate.[5] [edit] Repercussions A single event is not enough to cause damage to the joint, although there is a hypothesis that prolonged joint stress due to cracking knuckles may eventually lead to a higher risk of joint damage. However, the long-term consequences of this practice have not been studied thoroughly, and the scientific evidence is inconclusive. The common parental advice "cracking your knuckles gives you arthritis" is not supported by any evidence, but habitual knuckle crackers are more likely to have hand swelling and lower grip strength.[6] [edit] References 1. ^ What makes your knuckles pop?. HowStuffWorks. Retrieved on 2006-09-08.

I think it is due to cavitation in the joints - gases that are absorbed into the fluids in the joint come out of solution to form bubbles, then rapidly collapse, creating a 'pop' or 'crack' sound. As far as anyone can tell, it doesn't 'actually' do anything bad to your joints - the whole 'cracking your knuckles gives you arthritis' would seem to be a myth.

when you crack your knuckles or any other part of your body the air or "gas" excapes from the gap when the two bones join thus making the popping sound