A question that is often asked. It's called joint cavitation and it’s the sound that usually accompanies a spinal or joint adjustment/manipulation. Simply explained, when we safely separate the surfaces of a joint, a gas bubble forms in the joint space; this is what makes the popping sound. The attached post give a great run down on the mechanics and some research behind the phenomenon.
If you notice joints in your own body that pop or crack, as long as it is painless it’s usually normal and no cause for concern. If you have repetitive popping noises coming from your joints it’s more often other soft tissue structures moving around and that may warrant further assessment.
A final word - many people move their necks around to cause a joint cavitation; while this can feel like it gives a good release, I do not recommend doing it on purpose. This kind of self manipulation will usually release only the least resistant joints, not the tight ones, and you may end up with neck pain or headaches.
Hope you enjoy the article and have a great weekend.