There was an article at The Washington Post titled “Scientists are slowly unlocking the secrets of the Earth’s mysterious hum“.
National Geographic published an article on the same subject as well.
Both articles were reporting on the findings of this research paper * : “First Observation of the Earth’s Permanent Free Oscillations on Ocean Bottom Seismometers” which explains that the Earth’s hum is the permanent free oscillations of the Earth (Earth’s eigenmodes) recorded in the absence of earthquakes, at periods above 30 s (ultra-low frequency).
According to the research mentioned above the frequency of Earth’s hum is between 2.9 and 4.5 millihertz. The frequency is too low to hear. Human hearing threshold starts around 20 hertz.
Few quotes from the National Geographic article:
“The planet’s hum has tickled scientists’ minds since 1959, but definitive research was not completed on the topic until 1998. This current paper is the first to document the rumble from the bottom of the ocean. The researchers, spread out across earth science institutes in Paris, Stuttgart, and Oxford, U.K., combed through 11-months’ worth of seismometer records covering a patch of more than 1,200 square miles on the Indian Ocean floor east of Madagascar. From that data, gathered from 2012 and 2013, they homed in on two stations with high quality data and sliced out any interfering sounds from loud waves and electronic glitches. Behold: the native sound of the Earth.”
“By studying the Earth’s hum signal from ocean-bottom stations, scientists can map out a detailed landscape of the Earth’s interior. Currently, they can only look at the inside of the planet during earthquakes, which limits studies to certain times and areas. And when looking at seismic activity from land monitors, researchers can’t chart places far removed from islands and land masses. But the hum signal, droning and constant, can be detected across the world.”
* You can reach the paper at this url as well: https://marthadeencom.files.wordpress.com/2018/05/grl_deen_2017.pdf