I take a skepticism and logic class in school, it's a class about crazy things that happen, why sertain things happen, etc. On the first day my teacher brought up sounds, hearing sounds. He said sometimes when you hear something or your used to hear something, you hear it in your head.

Ex: If your used to hear a cellphone vibrate, sometiems you may hear or feel it, but you check your phone and there's nothing there.

My teacher also said it may happen with family voices, ringtones...basicly things you are used to hearing or used to hear for a long time then it just stopped.

Hope that helped a little.