What is this sound in the back of my neck????

It sounds like your neck "pops or cracks" in the morning, like some people can crack their knuckles or other joints. The sound simply comes from aculated ges being released when the vertebra moves back into its natural positiion. If you have pain, or even if you don't, it could be what's called a "subluxation." This is a misalignment that could eventually cause you some trouble or pain as you grow older. Go to a good chiropractor. They will take x-rays and show you the results (and explain them thoroughly) so you can see if that's the issue or not. If it is, a simple adjustment can be very beneficial. (I go every other week.)