Some just make my mind boggle rob!
What people were speculating the sounds are online...in 2015
Possible theories include:
Tectonic plates grinding - Tectonic plates are pieces of the Earth's crust and uppermost mantle, together referred to as the lithosphere. The plates are around 100 km (62 mi) thick and consist of two principal types of material: oceanic crust (also called sima from silicon and magnesium) and continental crust (sial from silicon and aluminium).
Atmospheric pressure - Atmospheric pressure is defined as the force per unit area exerted against a surface by the weight of the air above that surface.
Trains shunting - Self explanatory - noise comes from trains in reaction to the track and overhead wires.
Construction - Building works, especially if going on at the same time across a specific area, can led to similar sounds.
Aliens - Can this be an alien lifeform in the sky, perhaps scouring out Earth?
HAARP weapon - Rumours persist that the U.S. government uses secret weapons in the sky for defence and weather modifying, the High Frequency Active Auroral Research Program (HAARP). This wouldn't explain the sounds in other countries however...
The Apocalypse and the Seven Trumpets of Heaven - Seven trumpets are sounded, one at a time, to cue apocalyptic events that were seen in the vision of the Revelation of Christ Jesus, by John of Patmos. Somewhat more worrying as it would signal the end of the world...
Geoscientist David Deming from the University of Oklahoma has previously written about a phenomenon called The Hum – ‘a mysterious and untraceable sound that is heard in certain locations around the world by two to ten per cent of the population’.
Writing in the Journal of Scientific Exploration, he said that sources of The Hum could include telephone transmissions and ‘aircraft operated by the U.S Navy for the purpose of submarine communications’.
So what does NASA say?
According to Nasa, the Earth has ‘natural radio emissions’.
The Agency said: ‘If humans had radio antennas instead of ears, we would hear a remarkable symphony of strange noises coming from our own planet. Scientists call them "tweeks," "whistlers" and "sferics."
'They sound like background music from a flamboyant science fiction film, but this is not science fiction. Earth's natural radio emissions are real and, although we're mostly unaware of them, they are around us all the time.’
For instance lightning can produce eerie-sounding radio emissions, Nasa added.
Earthquakes can also produce sub-audible sounds, according to seismologist Brian W Stump from the Southern Methodist University in Dallas.
www.dailymail.co.uk/travel/travel_news/article-3084260/What-strange-sound-sky-Noise-heard-globe-nearly-DECADE-explanation.html