I'm watching this video called "Noises in the Sky" and my mother is terrified. I'm just like, thats clearly the sound of the city. I hear those noises almost all the time. You also gotta remember there are underground areas, and those areas echo, so you will hear noises coming from there, too. If there are lots of repairs going on in your city, you are 1,000 percent likely to hear strange noises.