From what I learned- when the ears can't hear a certain frequency well- because it's too high/low pitched (there could be loads of reasons, like hearing too much loud music with or without headphones, but a person's hearing deteriorates with age- it usually begins to deteriorate in the third decade of life but nowadays with young people it begins earlier because of loud music and such). So what the brain does- it makes you hear a certain "ringing" sound in your ears, instead- to make up for the sound that your ears can't pick up.

And BTW- I can hear it too sometimes, in my right ear mostly