I'm fortunate to have really terrific hearing. My family told me from when I was little that I took after my maternal grandmother, who they swore "could hear the grass grow." When I was in my early 20s I worked in a science research lab in a room that was custom-built to be soundproof so that I could make notes on tape-recorded animal noises. I kept complaining to my boss that the background noise (when the tape recorder was off) was annoying, and he swore that there couldn't be any background noise in this special room. Finally he decided that what I must be hearing was Brownian motion, the noise of the random movement of particles in the air. It still bothers me at night sometimes, when the whole house is quiet. It could also be the sound of the blood flowing in my ears, I guess, which might be more likely. Luckily, it isn't loud enough to be distracting.
Of course, I also hear sounds in the house that no one else hears, so it's always up to me to find the thing that is squeaking (an automatic timer alarm on our dishwasher) or to figure out what that annoying soft bang is (ice cubes dropping into the box in our freezer from the ice maker). Our son also has exceptional hearing, so when he was little we had to go outside the house if we wanted to talk about something we didn't want him to hear. And don't get me started about the awful noise that snails make in the garden when they're chewing our leaves - ugh!