<s>[INST] Hi Doctors, I hear and feel a fizzing noise at the back of my head/ base of my skull. It has been happening for about2 years. I have seen many other message boards with other people that experience this but have not seen a doctor address it. What do you think this is? [/INST] Hello, It is known medically as tinnitus and is the perception of a sound in the head or ears. There are many medical underlying conditions that may cause tinnitus such as ear apparatus diseases, neurological conditions, adverse effects of Chat Doctor.  An evaluation by an ENT Doctor may give further information about the cause. Hope I have answered your query. Let me know if I can assist you further.  </s>