Did you know that the crack that you hear when being adjusted by a Chiropractic Doctor is not the result of a bone snapping or cracking, nor is something tearing or breaking? That noise is actually the sound of nitrogen (N2) escaping a space. Think of the sound made when you open a can of pop. That is the sound of gas (CO2) being released from inside the can. Well the same thing occurs when your body is readjusted: trapped air gets released. Fixations (as they are called in the Chiropractic world) occur when a bone (or ligament, tendon, muscle etc) has shifted to the wrong place. You may experience pain, decreased movement, a tight feeling, or maybe nothing is felt at all due to a fixation. However, it is believed that when the body is properly aligned and free from fixations, that blood flows smoothly, muscles contract regularly, nerves are free from impingement, and toxins are drained with ease via the lymphatics. The body is overall healthier.
About that little crack... well, when bones are shifted, a new gap is formed and air can get trapped in this little pocket. That's why the crack is the hallmark of the Chiropractic profession. If the doctor has done their job correctly and removed the fixation, the crack (i.e. gas release) should be heard during the adjustment.
So don't be too afraid. A little gas release never hurt anyone ...right? :P