I have a concussion and now i hear a clicking in the back of my head. what could this be?

TMJ: You are probably hearing a click from the subluxation your tempromandibular joint. Consult your dentist for help with this.

Other things: Hearing problems can also come from changes in the air sinuses in your skull, such as with increased fluid or with a fracture. Rarely, tinnitus can be in a pulsatile fashion following the development of an abnormal communication between an artery and a vein after trauma.

Concussion: Since a concussion is related to trauma the trauma can effect other areas such as the TMJ joint and the neck.