When an adjustment is given it may result in the release of tiny gas bubbles between the joints that may sound like your spine is cracking or popping. The popping or cracking noise is caused by the change of pressure within the joint caused by the gas bubbles being released.

There is a fluid in your joints called synovial fluid which is a lubricant that nourishes the joints.  This fluid contains gases such as oxygen, nitrogen and carbon dioxide.  When a chiropractic adjustment is made, it stretches the joint capsule and gas is rapidly released sounding like a pop or crack.  After the adjustment, the synovial fluid re-lubricates the joint and normal motion is returned.

Not all adjusting techniques produce this noise as some adjusting techniques use little force and produce no noise at all.