Why is there this clicking in my head?

When you turn your head it takes several muscles working in conjunction with the vertebrae and intevertebral discs in your neck. So there can be several explanations for this. It could be a bit of arthritis in your neck or a slightly out of position disc. Most people do hear some sort of noise when turning their heads. Most people hardly notice it. I have it from time to time. With me I believe it is a bit of arthritis. As you say it is not painful and does not cause any other problems except for a slight noise when I turn my head. So unless the noise gets really annoying or starts to cause difficulties in turning your head I would not worry about it.