Welcome to HCM
We understand your concerns
I went through your descriptions. If you are sure that you can distinguish the symptom as it is, I suggest you not to worry too much and don't be too anxious.
People suffering from mental disorders named Hallucination
do experience hissing or fizzing noise from their head / inside head etc. Not only that , they also might experience so many noises. I am not suggesting anything. For the time being believe that you are healthy and do not worry about the noise.
Hope this clears your doubts. Available for further clarifications.