Is it possible that this could be temporary?
You could have tinnitis, as the others mention. It is pretty common.
But, if I were you, I would go to an Ear, Nose, and Throat specialist (ENT doc) and have a thorough exam. You say a couple of things that make it sound like you could have fluid in your inner ear or something else affecting the inner ear.
You can control the pitch by moving your head. (Maybe fluid is shifting.)
You have a feeling of fullness.(Fluid?)
You experienced some unbalance.(The inner ear affects balance.)
It comes and goes.
It also could be a number of other problems.
Seeing a qualified ENT doc is probably the best way to get to the bottom of the hissing.
Best of luck.
Well for me, it was the start of Tinnitus....
welcome to the world of aging....