That said some of the sounds on the tape are exactly what I heard. It's been twice now. The second wasn't as loud as the first.
So what I've done is to keep a mike keyed in one of my production studios. If the sound happens again I'll trip the Orban recorder as well as Cool Edit and a Reel to Reel tape. (If I can' find one, LOL! Haven't used them for YEARS!)
With my luck it won't happen again.
One other remote possibility is that there is a train tunnel quite deep under our building. It isn't used very often anymore and I am used to that sound as well. It really isn't a sound, more of a gently shaking. Plus I would have seen a train.
The Salvation Army is next door but there's no one there tonight. Across the street is a computer data storage/commercial IP provider but it doesn't look like there are a lot of people there tonight and they keep the door locked. I can't leave my studios long enough to go ask them if they heard anything. They'd probably think I was imagining things anyway...sigh.
Hope it isn’t the sound of a very deep sinkhole forming. ;^(
Good hunting! If you manage t get a recording, is there any way you can post it here? If so, PLEASE ping!
I spend a lot of going-to-sleep time listening to music—instrumental & non-vocal human—that I can NOT possibly record.
It isn’t exactly in my head, but (especially in the dark) when it is “quiet”, fridge/freezer motors, heaters, fans, etc cause faint vibrations/echoes to create harmonic echoes that my brain interprets as harmonies. I can move my head to vary the volume, pitch, etc, as the minute sounds change with the angles to the room’s corners. Ear plugs kill the effect, so there is a real physical component.
The framing & hollow sheetrock interior walls may act as an amplifier of sorts.