This is called Tinnitus and is typically caused by wearing earphones to loud, or working in a very loud area, or going to too many death metal concerts. You can do some things to help, but you can't fix it. Too late for that.
But I took note of a comment by William Shatner (the original Capt"n Kirk) who said that he had ringing in the ears for years and that it almost drove him insane.
He said that he eventually found a doctor that was able to help him greatly.
I always thought that if I met someone with that issue (and you obviously qualify) that I would suggest he start looking with that little bit of information that may help to narrow down a search for answers.