Some people can hear things that others cannot. Some are oriented towards hearing lower frequencies with a loss of higher frequencies, some can hear higher frequencies with a loss at lower frequencies, and still others have "gaps" where they cannot hear certain frequencies at odd points in their frequency response range.
Some sounds can be heard in some places, but not other adjacent places. For example, a few years ago there was an earthquake along the US east coast centered on Hillsville, VA at a magnitude of 5.2. It ran all up and down the east coast, transmitted through subsurface bedrock. I was asleep on the sofa in the living room, in Burlington, NC, and the missus came rushing into the room all agitated asking "what is that?". I opened one eye and said "relax. It's just an earthquake" and went back to sleep. You could hear it, feel it, and see things moving due to it inside the house, but when I asked folks about it that had been outside at the time, they experienced nothing. I personally think that structures such as houses may act as a sort of "sounding box" or "resonating chamber" as is found in some musical instruments that serves to magnify the sound inside them.
This house I live in now occasionally "jitters" - you can feel it shake slightly as if bumped by an outside force, as if a truck backed into the corner of the house and bumped it. I attribute that to the fact that a coal mine runs directly beneath it, perhaps in company with the fact that I live on an earthquake fault line. We often have tiny microquakes here that are barely perceptible.
One night years ago, I was out roaming the mountains in the dark when I encountered a specific point that I could hear a "hum" along with "clanking" noises (a rhythmic, metallic clanking like a blacksmith beating out a piece of hot iron on an anvil) that seemed to emanate from the earth itself, directly below my feet. One step forward, and it would disappear, and likewise for one step backwards from that specific spot. It could only be heard while standing in that one spot. There were no coal mines in that area, and nothing underground other than natural caverns that ran through the area making it like swiss cheese - solid limestone with random voids in it. I've never discovered the source of that noise.
The hum people are hearing may have some, or all, of those factors contributing to what they are hearing, with no locatable source... but I have no doubt that what they are hearing is real.
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Some sounds can be heard in some places, but not other adjacent places. For example, a few years ago there was an earthquake along the US east coast centered on Hillsville, VA at a magnitude of 5.2. It ran all up and down the east coast, transmitted through subsurface bedrock. I was asleep on the sofa in the living room, in Burlington, NC, and the missus came rushing into the room all agitated asking "what is that?". I opened one eye and said "relax. It's just an earthquake" and went back to sleep. You could hear it, feel it, and see things moving due to it inside the house, but when I asked folks about it that had been outside at the time, they experienced nothing. I personally think that structures such as houses may act as a sort of "sounding box" or "resonating chamber" as is found in some musical instruments that serves to magnify the sound inside them.
This house I live in now occasionally "jitters" - you can feel it shake slightly as if bumped by an outside force, as if a truck backed into the corner of the house and bumped it. I attribute that to the fact that a coal mine runs directly beneath it, perhaps in company with the fact that I live on an earthquake fault line. We often have tiny microquakes here that are barely perceptible.
One night years ago, I was out roaming the mountains in the dark when I encountered a specific point that I could hear a "hum" along with "clanking" noises (a rhythmic, metallic clanking like a blacksmith beating out a piece of hot iron on an anvil) that seemed to emanate from the earth itself, directly below my feet. One step forward, and it would disappear, and likewise for one step backwards from that specific spot. It could only be heard while standing in that one spot. There were no coal mines in that area, and nothing underground other than natural caverns that ran through the area making it like swiss cheese - solid limestone with random voids in it. I've never discovered the source of that noise.
The hum people are hearing may have some, or all, of those factors contributing to what they are hearing, with no locatable source... but I have no doubt that what they are hearing is real.
.