Sounds associated with joint motion are usually of no great importance unless there are other symptoms. For example, a cracking sound from the knee when going from a standing to a squatting position is unlikely to matter much if it is not accompanied by pain, limitation of motion or swelling. The same goes for cracking knuckles or crunching noises in the neck.
The neck has an intricate arrangement of bones, joints, ligaments and muscles that allow the upper spine to turn, bend and rotate while protecting the spinal cord and other nearby structures. Some people notice noises or a grinding sensation in the course of normal neck motion.
Of course, crunching or grinding noises during neck motion that is associated with other symptoms could be important. For example, such noises accompanied by neck pain, arm weakness or numbness could be an indication of arthritis, disc disease or a combination of these.
If you are concerned about the noises you're hearing during neck motion, see your doctor. While it's unlikely to be due to anything serious, the reassurance may be worth the visit.