What makes that popping sound during an adjustment? 
Chiropractic adjustments are a safe and useful tool for providing pain relief, while helping to reduce joint and muscle stiffness. For anyone who has experienced a joint manipulation you may have noticed an audible “pop” or “cracking” sound that follows the adjustment.
What exactly is this sound?
For starters, this “Pop” sound is called a ‘CAVITATION’. It is NOT created by joints moving back into place, or bones grinding on top of each other. This audible phenomenon is created through a chemical process called TRIBONUCLEATION.

Tribonucleation is a process that describes a sudden decrease in the pressure of a capsule – in this case a joint – causing dissolved gasses to be rapidly released into the joint cavity. The rapid separation of the joint which typically occurs during a joint manipulation creates this gas cavity to form, thus creating an audible ‘POP’.
Check out this video of a finger undergoing a joint manipulation while in an MRI machine. Watch as the gasses form within the joint! (Click image or link below).