What's the popping/cracking sound?

When an adjustment is delivered to the spine, the popping or cracking sound that frequently emanates from the joint is, from current research thought to come from release of gas. It is NOT coming from any cracking of bones. In fact, if you consider your knuckle joints, once you crack them, it takes a while for them to crack again. Spinal joints, while requiring a highly trained specialist to manipulate, keep their adjustment far longer. A joint can only "Crack" or "pop" if it's LOCKED UP and not moving normally! Even a DC cannot make a moving joint produce that noise.
So, it's a really good sound and thus good sign when it makes that sound. All manipulations do not make that noise though, and one should not judge the efficacy of a treatment by the degree of noises coming one's spine.