Did you ever work or do chores near anything around the volume of a lawnmower? Older lawnmowers were JUST over the limit to damage hearing (without being even close to loud enough to cause pain), so that kind of thing could cause seemingly random tinnitus. It's why lawnmower manufacturers worked hard to reduce the noise--that 5-10 decibel reduction was crucial to prevent newer generations from getting tinnitus.
I only know this because it's exactly what happened to my mother. She worked in a school and never flew anywhere, went to any concerts, or listened to much music. Her mowing the lawn was the sole cause. (It didn't help that she didn't use any hearing protection.)