The podcast is called Crosscurrents, and the episode is called, "The Hum, worldwide mystery sound explained." What's common about them and what makes it the "worldwide" Hum is that these hums are all low frequency, or low in pitch. Low frequency sounds have a similar throbbing characteristic that can be annoying even when it's quiet. Low frequency sound also travels further and is able to pass through walls and ear plugs more easily than other sound. "The most plausible source for this widespread infrasound and low frequency noise pollution is high pressure natural gas pipelines.
