Perhaps you've already considered this, but could it be caused by a fan or some other source of mechanical vibration in your house? Acoustic propagation within a house can be surprisingly complicated.

Some years ago I noticed a very strange hum in one corner of my basement. Thanks to funky acoustics, I couldn't identify the source for a while, but IIRC it ended up being a fan in another part of the house. I suspect most of the energy traveled through the house framing, and ended up resonating in the room I was in at the time.

It’s very possible. There’s another source of noise that I’ve noticed, it’s qualitatively different so I don’t think it’s related, but there is a life flight helicopter pad about 2 miles from me. Every now and then there will be a real low rumble in the house that you can actually feel more than hear. About a minute later the helicopter will fly overhead.

There is an auto assembly plant about 5 miles away, if it’s not in my head it’s almost certainly something from there. The ground here has a extremely high clay content, and when you get down a bit it’s like one solid mass. It seems ideal for transmitting sound long distances.

If you have enough time when you notice the sound, it might be interesting to turn off your house's main circuit breaker to see if the sound goes away. Might be a quick way to bisect the analysis.

Ah! Now that you mention it, I have done that. It didn’t make a difference and I think that is roughly when I started seriously considering this was all in my head.

I can see why that possibility would drive you nuts!

Then I second the idea of getting two recordings (baseline and when you hear this) using a suitable mic. If you can afford the hardware, it's got to be way cheaper than questioning your hearing and/or psyche for the rest of your life :)

Definitely worth trying! Thanks for the ideas and the support, 2020 isn't the best year for 'hearing weird things at home' haha :)

Glad to be a sounding board. Yeah, 2020 is definitely the year we could write the book, "Gaslighting Yourself for Dummies". Best of luck :)

Perhaps you could clamp a microphone to a metal rod stuck into the Earth to test that hypothesis.

Same for clamping onto the structure of your house for the vibration/fan one?

Finally, if all else fails, clamp a microphone to your head

I could hear a fait beeping in one corner of my kitchen, but if I moved ~3 feet in any direction it disappeared.

Eventually discovered that it was some kind of acoustical beat frequency from the motor noise in the refrigerator reflecting off the back wall. Moved the refrigerator ~1 inch further away from the wall and the sound disappeared (at least from the location I had been able to hear it)

I heard an annoying hum out in the wilderness of Charlo MT that nobody else could hear, but chalked it up to radio equipment because there WAS actually a large tower nearby, that could be explained as part of a hi-def video streaming operation to capture these lovelies, just "next door" aka within a few miles.

Charlo Osprey Cam: https://explore.org/livecams/ospreys/charlo-montana-osprey-n...

I assumed there was some heavy duty radio equipment to handle this but was never really able to confirm the source of the noise, I was visiting for a couple of months two years ago. There were days when I just could not tune it out, and there were days when I either didn't notice it at all, or it wasn't present.

Towers can generate noise all by themselves in moving air through vortex shedding.

How do you know the frequencies they're at? Did you Match it with sounds from your computer?

What constitutes high quality microphones for you? Where these microphones that were made for this low frequency range?

EDIT: Especially on the microphones maybe the actual noise isn't 28Hz, but the 28Hz are harmonics of a 14Hz, 7Hz, 3.5Hz,... vibration. These harmonics may get produced inside some structures near your ear, explaining why you hear it and others don't.

You'd need special equipment to measure these low frequency vibrations.

Yes matching sounds on the computer. I can actually hear a beat when they are close but not exact.

Mics were a variety of xlr and usb from rode, sennheiser and yeti over the past few years. Definitely in the frequency range but only moderate sensitivity at that freq and low to moderate self noise. Not exactly scientific but I would expect to see a bump of energy in the spectrum and get nothing.

Yeah you'd need PCB mics with reference level sensitivity. They cost over 800$ and the equipment that measures the noise costs in thousands.

I'd be really intrigued whether you can find something at the harmonics using an infrasound microphone (some can measure down to 0.1Hz), but the price is probably prohibitive for an experiment.

Saw some interesting research on using infrasound to locate elephants. they managed to come up with something that was about $9 in parts including microphone. (Plus a SBC for analysis).

A bit of searching lead to some DIY stuff using subwoofers (speakers) as the microphone input then some low-pass filtering and then fed to an op-amp.

Here's the link to the research paper on the elephant detector https://www.researchgate.net/publication/320596116_Eloc_Loca...

That's the next step, just have some other projects to get out of the way first. There's a fairly low cost infrasound recording option using loudspeakers[1] that I think is worth a try. I'm also interested in building a seismograph, which might be able to pick it up on the high end of its range. If the sound is anywhere as loud as I hear it, there isn't much need for anything ultra-sensitive.

[1] https://www.youtube.com/watch?v=mekmsgjSqR4

I actually plan on recording sounds from my head, too. Could be weird spasms or something causing it, the lack of any variation in the two pitches makes that seem unlikely but possible.

They might be able to do it with an Earthworks M30 measurement mic, which claims accuracy down to 3 Hz, and an appropriate digitizer. It's expensive but not as blisteringly expensive as lab mics like Brüel & Kjær's gear.

I wonder if fans with (not a power of 2) blades could also generate surprising tonal (i.e., not transient) frequencies.

I could imagine this logic also applying to other pumps in the house: AC pumps, baseboard heat circulators, sump pumps, etc.

I heard this weird hum in what I would judge as similar frequency band to yours when I stayed at my sisters new house. The thought that I have a weird new ear problem or something really freaked me out at first. And then I realized that I'm only sleeping on a thin mattress directly on the floor and they have a water circulating under floor heating there that just created this hum.