Inflammation. Shoulder "crunching" could be completely benign, especially if it is not painful or you are not limited by it. If it is painful you may have developed bursitis. Typically bursitis is treated with nsaids, specific rotator cuff exercises, sometimes an injection if the symptoms do not resolve on there own.
Snapping tendons. Most likely this is tightening of the tendons/capsule around the shoulder joint when you tense them, much like popping your knuckles. Usually not anything bad at all. Occasionally this will be from either rotator cuff problems or arthritis, but these would both be uncommon problems at your age.