1. Someone snorting mucus back up into his nasal passages
  2. Teeth on a fork
  3. A phone is ringing and it's not yours but no one else is picking it up
  4. Someone else is typing on a phone and has the sound on
  5. Any kind of audio notification on a computer, especially persistent ones like chat and email
  6. Your dogs barking furiously at nothing every freaking time someone walks by
  7. Teenaged girl shrieks at a Taylor Swift concert
  8. A baby crying in the distance (usually next door neighbours) and nobody seems to care
    Suggested by @henweb
  9. Phone vibration
    When people openly let their phones vibrate in a quiet environment and think you can't hear it.
    Suggested by @graceofwrath