[Bolts]

I've read most of the posts on the subject of valve noise, but I haven't found the answer to this question yet.
I would like to know what causes the tick sound you hear when a valve adjustment is slightly over the max, assuming everything else is working properly.
Does the noise happen when the valve face contacts the seat or does it happen when the cam comes in contact with the lifter or when the lifter contacts the valve stem? Or is it happening in the spring somehow?
Is a ticking valve something to really worry about?
Thanks.