Explanations vary as to why these gas bubbles form. Mostly, they are just a normal consequence of movement and it just so happens that we “hear” them in the neck and upper body because they occur close to our hearing organs. However, they can also be produced when we “crack” our neck vigorously which is something many of us do when we have tense neck muscles.
Now, cracking our neck to relieve tense muscles is usually fine but if we do it too often and too vigorously then problems can occur. When you crack your neck you risk stretching the ligaments which hold the neck vertebrae together. Then what happens is your body perceives that the neck joints have become slightly unstable and your muscles are tightened to compensate for the lax ligaments. Since you have tense neck muscles again, you might once more feel the need to crack your neck. And so on – a vicious circle is created.
So should you be concerned if you have crepitus? Probably not, but don’t get into the habit of violently cracking your neck (or any other body part for that matter) to relieve muscular tension. There are safer ways to ease tight muscles including bodywork therapy.