It is like in the back of the head inside the spine and brain area and it's a relfex
What makes you call this experience a reflex? This is not clear.
when activated creates a dark rumbling feeling in the back of the brain area. What is it ?
The only thing I can relate this to is the buzz-like sound I hear in my ears when I close my eyes very strongly. In other words I don't know what it is.
In sure I'm not the only one with this reflex but is it an unconscious mind placement? What does it do what is it's function?
How do you arrive at the conclusion that this is an unconscious mind placement? You seem to jump to conclusions.... Perhaps consider that this experience could be something totally 'innocent' and possibly 'meaningless'. It could be a perk of your body. What is relevant here is how you take information from your body and turn in into a speculative theory. This you need to investigate in writing with self-forgiveness.
When investigating a point in your body (or any point within yourself or within the world) you need to make sure that you are Breathing. This will prevent you from allowing your attention to become distracted with thoughts and imagination. If you allow a thought or imagination to tell you what is real, then you will in fact be 'fabricating' what you 'see' instead of developing real communication with yourself and your body.
On another nore - if you find that the point you are dealing with requires serious investigation, you can consult a medical specialist and/or get a scan done. This way you get neutral feedback.