Fizzing sound in back of neck, think its circulation of cerebrospinal fluid. Normal for it to be audible?

Not likely,,, This is highly unlike to be audible. It's deep on your brain and around your brain and contained in membranes. If you have this sound, it's likely to be something else like vertebral joint crepitus or sounds from joint movement, or even just the brushing of the hairs on your neck. If you're still concerned, you can see your doctor for an exam. Good luck.