For a good year I used to here this high-pitched whine when visiting my grandparents. I could hear it throughout the house, and though it was either a faulty vent, or seagull chicks on the roof. It was not constant, and rather intermittent.

I checked everything, but nothing. Asked my grandparents about it, but they couldn't hear any noises. Finally I got a friend over, and he could hear it clear as day.

One day I noticed my grandmothers hearing aids lying in a casing, and it turns out those were the source of noise. She'd forget to take out the batteries, and they would create a negative feedback-loop that came and went.