The noise that is heard during an adjustment is the release of air and fluid between the joints of the vertebrae(spinal bones).  Most people think it the bones moving on one another and that is not the case.  If that were to happen, the adjustment would be very painful.  When the bones are gently adjusted, air and fluid are released, and the popping noise is heard.  During this process, pressure that has been damaging a nerve is released, allowing the nerve to send healing energy to the painful site.