For The Past Year I Have Had A Grinding And

For the past year I have had a grinding and cracking noise at the base of my skull as if sand is between my bones, it's gotten to the point where it's starting to hurt. I have been to several doctors and all have told me it's due to stress and my school bag but now that's it's summer I know for sure it is not because of either of these reasons. I just don't know what causes it ?