What's that mysterious ringing in your ears?

In many people, tinnitus has been linked to hearing loss – both age- and noise-related. — TNS

Tinnitus – the sensation of hearing a sound when no external sound is present – is often described as a ringing, buzzing, roaring, clicking, humming, pulsing or hissing sound, or any combination of these sounds.

You may hear it in one or both ears. The sound may be present all the time, or off and on.