well is it constant? and can you hear it only when there is complete silence? if it's not constant and you only heard it in silence it may just be background noise made from machines i have been hearing it from when i was a little boy and i can not hear it when i am camping
if you are hearing it constantly than seek medical help
Rule out the mundane first. Tinninitus can produce a rather obnoxious ringing. If all health problems are ruled out, you may be sensing spiritual energies. I for one hear the sound of cicadas when a spirit is present.
probably tinnitus, rather common. it's a high pitched ringing that normally occurs after listening to loud music. could also be some background sound. my new place has rads and they emit a high pitch humming noise when they're on [normally i hear them in the morning and at night] so if you hear it at around the same time it's probably something like that. go to get your ears checked out, could be something wrong with them. have you cleaned them recently? too much wax in your ears can effect your hearing. pour a little bit of hydrogen peroxide into them [about a cap full] pour it into one ear and leave for about a minute, then turn your head over to let it drip out [repeat for the other side] finally clean with a q-tip.
if everything checks out and you still hear the ringing, a spirit/demon could be trying to speak with you, and while you have some clairvoyant abilities, you may be unable to hear them, so it's coming across as a humming noise. i can hear spirits clearly so i can't tell you from experience if that's what it is.