After a while, you kind of get used to it. It's a hum that some have compared to white noise but its origin is a mystery to many Missouri residents who keep hearing a low repeating noise.

Get our free mobile app

Fox 2 St. Louis reported on a phenomenon mostly being reported in north St. Louis County. The airport says it isn't them. Same for the power companies and other utilities. It remains a mystery for where this sound is originating, but it's lasting for hours according to residents.

You might remember that last summer we reported on mysterious loud booms that were heard in Missouri. While some suspected sonic boom from aircraft, that could never be confirmed. There was one in December that was a military plane, however.

In the case of what's being heard between our part of Missouri and St. Louis, it's much more mechanical and almost drone-like. If you're prone to enjoying conspiracy theories, there are some that believe there are tunnels being worked on underground in Missouri and other states for deep underground military bases. Tin foil hat required for that one, though.

According to the Fox 2 Now story, the mystery is now being investigated by the US Army Corp of Engineers. Until they come to a conclusion, the source of this weird low repeating sound remains a mystery.

READ ON: Weird, wild UFO sightings from throughout history

LOOK: Stunning vintage photos capture the beauty of America's national parks

Today these parks are located throughout the country in 25 states and the U.S. Virgin Islands. The land encompassing them was either purchased or donated, though much of it had been inhabited by native people for thousands of years before the founding of the United States. These areas are protected and revered as educational resources about the natural world, and as spaces for exploration.

Keep scrolling for 50 vintage photos that show the beauty of America's national parks.