My first experience with this strange, low frequency hum was sometime back in 2001 in New Jersey. I was in my first home, alone, sitting up in bed one night reading a book. It was very quiet, and I started noticing a very low droning noise. And I mean a very low hum not some ominous trumpeting sounds from the sky. This was a low, persistent sound.
Someone posting on a forum I visited while researching the hum described it like a tractor tailer climbing a hill in low gear and never reaching the top. That’s the most accurate description in my opinion, just a continuous motoring sound.
Similar mysterious hums have been reported by people around the world, “the Taos hum” in New Mexico for example, and other places.
The sound I heard was so low and monotonous, unordinary, that I assumed there must be something going on with my ears. I tried the usual pressure relief trick, holding my nose and creating a slight pressure to “pop” my ears, but the sound persisted, so I began making deliberate noises to see how the hum would be affected.
If anyone was watching they would have thought I was nuts. I’m sitting there in bed with a book on my lap blurting out calls of nothing in particular, “HAP! HOOOO! PSHHHH!” And making throat clearing noises. After each noise the hum would promptly return.
I spent some time looking into it, but didn’t give it too much further thought and more or less forgot about it. Since then I have noticed the hum periodically, and not just in New Jersey. I heard it while on vacation in North Carolina last year. That time my wife was beside me in bed and I asked if she had heard it too. She claimed not.
We’re living in Southeastern Pennsylvania now, and the most recent occurrence of the hum was just last night in bed, and this time my wife did hear it!
I had just switched off the light and we were laying there in the silent darkness. A few moments went by and I said, “I hear the hum again.” Then I went on to reproduce the pitch I was hearing in my head by humming a low tone.
She said, “I hear it too.”
To clarify I said, “not the hum I just made, but something else…”
She replied, “I heard it before you even said anything.”
I was ecstatic, “You did?! You heard THE HUM?! Can you hear it now?”
We were quiet for a moment and, just as the hum returned to my attention, she said, “Yep, there it is. The same low sound like you just made. Sounds like a truck on the highway.”
I said, “Yeah, but if it’s a truck on the highway it would have driven off by now. This keeps going.”
She agreed. We discussed the sound for a few minutes, between silent listening, and eventually drifted off to sleep.
This morning I came down to my studio to work. I’m a musician and was preparing to work on some recording. While I waited for my computer to boot, a relatively quiet Mac Mini, I heard the hum. That was about 9:30 this morning, May 19, 2014.
I had a cardioid vocal microphone set up so I decided to record the silent room and see if I could hear the hum in the subsequent recording. After recording for one minute I analyzed the recording with an EQ set up on the audio track. I increased the gain 20dB, cut all frequencies above 200Hz then, while the sound was playing, swept a very narrow band pass slowly from 20Hz up to hunt for the most resonant frequencies.
Tones were present at 60Hz, 90Hz, 120Hz and 180Hz, the strongest of which was at 120Hz and the second strongest at 60Hz. Based on these frequencies the origin in this case could be electrical, however the frequencies were not present in a recording with the microphone input off, so they are not originating in the equipment.
The HVAC system is off, so the only motors running in the house are a radon pump in the attic, the refrigerator and a small impeller pump and air bubbler on the fish tank. The fridge does not run constantly, the fish tank pumps are relatively quiet…
Oh crap, I think I know what the hum is: 3.5 inch Hard Drive in an external USB enclosure (metal housing), sitting on my wooden desktop. Have to test that theory later this evening when my wife is home. But to be sure, I just lifted the drive off of the desk and the hum went away :)
UPDATE: I tested the theory last night. The hum was still present, two floors up with the computer and hard drive off! The hum we are hearing is not caused by vibrations of the hard drive resonating on the desk. It’s something else. Our investigation is ongoing…