The hum has been a subject in Northern Earth magazine for several years – see for example page 25 issue 151, December 2017. At https://phys.org/print431859742.html … it seems earthquakes can cause the Earth to vibrate for an extended time but in 1998 researchers found the Earth itself generates a low frequency vibrational signal even in the absence of earthquake activity. This has now been extended to the sea floor. Seismometers on land and under the sea have determined the ocean bottom naturally vibrates – a low hum like a wind turbine.
The hum has even been found in zoos at night. Angela Stager at the University of Vienna picked up a humming coming from the giraffe enclosure, in three different zoos, at night (which was otherwise still). The hum was a low frequency sound of about 92 heta, just inside the human audible range (quoting New Scientist 17th September 2015). Subsequently, neighbours of Paignton zoo in SW England complained of a humming noise and it too was found to be coming from the giraffe enclosure – and a 165 signature petition was presented to the authorities. Coucil officials suspect it may be something quite simple such as a cooling system that is responsible. Why wouldn't they. They went on to check out the heating system and various other possibilities but found nothing. The neighbours are not happy. The report by Paul Screeton then said another, later, news item claimed researchers believe micro-seismic activity for ocean waves (and Torbay faces the Atlantic) affecting the sea bed and the land produce the hum. This is one of the theories explored in the PhysOrg piece – but was discounted. What is going on? Is it like the hum of an overhead pylon?