Not all adjustments and manipulations make a sound. The goal of an adjustment is to increase the mobility and flexibility of a joint. While many manual back or neck adjustments can make a "crack" or "pop" sound, others adjustments do not. Of the many ways to adjust and mobilize a joint, the most well-known "non-cracking" treatment involves the activator or impulse instrument adjusting tools. These, and other, low force techniques can be incredibly effective on many people, and allow for joint and muscular adjustments that do not "pop".
It is important to remember that the cracking sound that can happen from an adjustment is gas releasing from a change in pressure in joints. In our opinion, these sounds happen incidentally, and are not a requirement for treatment to be effective.