13. The Hum
The Hum has no particular place of origin. Populations in towns across the United States and the United Kingdom have reported hearing a low rumbling, like the dull roar of an engine. The sounds have not been explained. People hear the sounds for a period of time before the Hum drops off.
mic.com
Not everyone in the area reports hearing the Hum, so many question its very existence. It is reported that it could be a widespread case of tinnitus, caused by some unknown force. Other explanations include, fish, spontaneous noise emissions, or unreported uses of heavy machinery.
strangesounds.org
Call me