A long time ago I heard clicks when I jumped and walked; any vertical movement of my head did it. It really freaked me out; I never did find out what was making that noise.
Heh, if I "press" my head down slightly.. on its own.. wait, what? How the heck can I do that?? Um, okay, apparently you can use your neck muscles to make your neck somewhat shorter? At least it... feels like that... er... anyway...
If I do that, OR tilt my head to the right and rotate it to the left, the back of my neck goes "crack crumble" really silently. Like... like stepping on bread crumbs. It's quite annoying. But the only time it happens is if I want it to. Or if I suddenly look at something on the floor to the left of me... which doesn't happen very often.