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.
I'll let it run and review the audio when I get home or tomorrow night if I can't email it to myself. If there's anything there I'll post it. Starting to get too busy now to review it this morning. I'm getting ready for the morning shows.