You may have tinnitus. I have it myself. Tinnitus is basically a phantom sound you hear because of damage to your inner ear. The damage causes you to lose a frequency of sound and because you can no longer hear that frequency you will hear a phantom sound that replaces the missing frequency. The sound can come in many forms, including hissing sounds.
I'd advise you to get a hearing test, that way it will be confirmed if you're actually missing a frequency which is causing the sound. Make an appointment with your doctor and they'll refer you to an ear specialist.
When i first had tinnitus I was really upset and stressed about it, as music is one of my biggest passions in life. I was worried I'd never be able to enjoy music again and never go to concerts, etc. That was a few years ago. Now I don't even notice it. In time the brain habituates to the missing frequency and you won't notice the sound anymore, unless you specifically listen out for it. It could habituate to it completely and the sound may never return. The main thing is not to worry. Stress will make the sound more audible as the blood pumps through the vessels in the ear, increasing the volume of the tinnitus. My advice is to go see your doctor and get it checked out. If they confirm you have tinnitus just try and relax and not stress yourself out about it. Just remember the sound won't be there in its current form forever, it'll improve. From what you describe, a hissing noise isn't on the high end of tinnitus phantom sounds, so you may only be missing a small frequency (if you do have tinnitus) which will be easy for your brain to habituate to. Good luck.
