Once you've passed your earlier twenties, you start to hear strange noises coming from your joints...snapping, cracking, popping, and clicking, and you start to wonder, "Is it all downhill from here?" Well, I won't answer that last part, but let's talking about the sounds first. There are a couple different ways that joints make sound.
1. The knuckle crack- a onetime snap, sometimes coupled with a feeling a release afterwards. This can happen with other joints too and is called cavitation. (More on that later).
2. The passive pop- a onetime pop that happens in the midst of activity. Maybe you do a twist and your mid-spinal vertebrae pop or you shake hands with someone and your wrist cracks (I say this from experience).
3. Clicking- a joint that perpetually pops, no matter what you do. Some folks can do this with their jaw or neck (yikes!).
4. Ball and socket joint clicking- For me, my femur (leg bone) doesn't always quite sit right in the acetabulum (the hip socket) so when I do circumduction (circular motion), there is clicking. This might also happen in the ankle, shoulder, and wrist.
So what does it all mean? Well, surprise surprise, there isn't as much documented research on this as one might hope, but let's proceed.
Cavitation: Joint capsules are surrounded with synovial fluid. Pockets of nitrogen gas can form a bubble, and the action of cracking may collapse the bubble. It takes anywhere between 20-30 minutes for the pocket to form again, which is why you won't get the same sensation if you try to crack your knuckles twice consecutively.
The passive pop can be activity based or chiropractor initiated, and in the case of the spine, often is a readjustment of a misaligned vertebrae. I also notice that my spine usually pops in the morning, either from previous activity or lack thereof. Some medical professionals have different theories as to what the pop is: some people believe it's the sound of ligaments snapping as the joint is adjusting, and the sound can be called "crepitus."
Clicking: so either form of clicking, either jaw/neck or ball and socket are things to be more wary of. In my experience, the repetitive clicking is not something to seek out, but instead, make smaller movements until the clicking disappears and then start moving more fully again. Are you still a little confused as to how circumduction might induce clicking? Here's a friend and YTU® teacher Alexandra Ellis teaching Propeller Arms, which is a dynamic warmup featuring shoulder circumduction. If someone was having perpetual shoulder clicking while doing this, it would mean that their arm bone might not be sitting in the socket well (amongst other things) and that the joint is unstable and not supporting the movement.
This is also my absolute favorite shoulder warmup, not only challenging your brain but your arms/shoulders! Give it a shot if your joints permit it.
Two caveats re: snap/crackle/pop-
Please don't violently crack your neck. Ever. It's just not a great idea because your cervical vertebrae are fragile and very mobile, the least "fixed" of your vertebrae. And if you have a clicking jaw, don't open your mouth to the point of the click/pop. It could be an early sign on TMJD. (When I started having jaw clicking about 10 years ago, I stopped chewing gum and the clicking stopped eventually).
If snap/crackle/pop is accompanied by pain, stop forceably cracking your joints, and see a doctor! Excessive joint sounds (that don't go away after a few movements) can be a sign of joint instability or an early sign of osteoarthritis, so pop with care.