Mystery high-pitched noise keeps Oregon town awake at night | US news | The Guardian:What's odd, as reported in the news item, is that the sound had appeared "decades ago" and is only heard at night. It was a mystery then and a mystery now. Before listening to the audio provided in the article, I thought it might be a bird call. But it's definitely a mechanical sound. To me, sounds like metal scraping against something. Whatever it is, it's a mystery.
"An unexplained high-pitched tone has kept residents of a Portland suburb awake at night for at least a week, confounding the best efforts of police and firefighters to pinpoint its source."