For me the first google result the Wikipedia page (https://en.wikipedia.org/wiki/The_Hum https://en.wikipedia.org/wiki/The_Hum) which offers several explanation

For me the first google result the Wikipedia page (https://en.wikipedia.org/wiki/The_Hum https://en.wikipedia.org/wiki/The_Hum) which offers several explanations, including actual measurable man-made external sources, a type of tinnitus, and some more rare or speculative circumstances. My understanding is that hum-like low-frequency tinnitus is not all that well documented. Feel free to provide links to sources to prove me wrong.