Why do i hear spinal fluid from the middle of the back of my neck to my brain stem?

Probably not: It is very difficult to hear spinal fluid of any type unless it is actually dripping out of the head-but then you would be very sick and have other symptoms. What you are more likely describing is a sensation of movement or vibration in the region of the neck. It is difficult to say exactly what you are hearing, but physiologically and anatomically what you describe is unlikely.