What Causes Fear And Anxiety In A Child?

My 4 year old son has recently started to become fearful at night of noises - clicking more than anything. When we listen hard, we can identify some of the normal household sounds, but not all the ones he can hear. he complains of distant music,that we can t hear, or if we can it is on the very edge of hearing. He suffers from an eternally running nose that we have recently been given a steroid based nasal spray for and he takes antihistamine, for the same reason. otherwise he is in good health - but becoming exhausted predominantly by the hard struggle we have in getting him to settle at night. He puts up an emotionally draining fight to avoid being alone with the clicking, and is becoming out of hand. do you have any suggestions or advise? Thank you