I got mild tinnitus as a kid, and I remember being scared about it because it happened all at once one night, going from nice pure quiet to what sounded like a very loud ringing in my head. It's almost like a constant single-frequency tone plus a slightly lower intermittant tone that sounds a bit like morse code.
It doesn't bug me except at night, but it's pretty irritating then. It does make passing my annual hearing test a bit tougher, but it's not getting much worse so it's merely an inconvenience.