The Taos Hum (from Wikipedia):
The Hum is a generic name for a series of phenomena involving a persistent and invasive low-frequency humming noise not audible to all people. Hums have been reported in various geographical locations. In some cases a source has been located. A well-known case was reported in Taos, New Mexico, and thus the Hum is sometimes called the Taos Hum. Hums have been reported all over the world, especially in Europe. A "Hum" on the Big Island of Hawaii, typically related to volcanic action, is heard in locations dozens of miles apart. The Local Hawaiians also say the Hum is most often heard by men. The Hum is most often described as sounding somewhat like a distant idling diesel engine. Typically "the Hum" is difficult to detect with microphones, and its source and nature are a mystery to the listener.
Then there's also the Mistpouffers:
Mistpouffers are unexplained reports that sound like a cannon or a sonic boom. They have been heard in many waterfront communities around the world such as the banks of the river Ganges in India, the East Coast and inland Finger Lakes of the United States, as well as areas of the North Sea, Japan and Italy; and sometimes away from water. Names (according to area) are:
- Around Seneca Lake and Cayuga Lake in the USA: "Guns of the Seneca", "lake farts".
- coastal North Carolina: "Seneca Guns"
- lower Connecticut valley: "Moodus noises"
- Bangladesh: "Barisal Guns"
- Netherlands and Belgium: "mistpoeffers"
- Italy: "brontidi" or "marinas"
- Japan: "uminari"
- Philippines: "retumbos"
- elsewhere: "fog guns"
They have been reported from Passamaquoddy Bay in southwestern New Brunswick, in Belgium and Scotland, at Cedar Keys, Florida, Lough Neagh in Ireland, Western Australia and Victoria in Australia, on an Adriatic island in 1824, at Franklinville, NY in 1896, and in northern Georgia. They have been heard frequently on calm summer days in the Bay of Fundy.