Why do you have a clicking noise in the back of your head?

There is no such condition as a "clicking noise in the back of your head." Your skull is a solid bone, and there are no moveable parts that could make a clicking noise.

If you think that there is a clicking sound in the back of your head, see a doctor at a hospital who can provide proper medical diagnostic tests and treatments options tailored to any health conditions identified. Self-diagnosis and treatment, including taking medical advice from random people such as myself on the internet can be unsafe and even life-threatening. Take care of your health by consulting qualified personnel.