High-pressure natural gas transmission pipelines have been identified as a plausible source for some Hum reports, particularly in areas where underground pipeline infrastructure is extensive. Natural gas flowing through pipes at high pressure generates low-frequency vibrations, typically in the 10-100 Hz range, that can propagate through the ground for considerable distances. These vibrations can cause structures built on the ground to resonate, particularly at night when ambient noise levels drop and the vibrations become relatively more noticeable.
In Bristol, England — where the Hum was first widely reported in the 1970s — major natural gas pipelines run beneath and around the city, and some investigators have suggested a correlation between pipeline routes and areas of concentrated Hum reports. A 2019 study in the UK examined the relationship between gas pipeline pressure variations and Hum complaints, finding a tentative correlation in some areas but not in others. The pipeline hypothesis is attractive because it explains the geographic specificity of some Hum hotspots and the sound's low-frequency, continuous character, but it does not explain why only a small percentage of residents in affected areas perceive the sound.