Had something similar happen (not that similar) with my wife. In the end it turned out to be a phone charger and every time someone plugged in their phone it would emit a high enough frequency she could hear it and I couldn’t (1 year difference in age). Probably not the same thing but think of anything electrical that is being used at those times. It is VERY hard to pinpoint the direction of that sound and if she gets it in her head that it’s coming from the wall it’s impossible to hear it from anything else. My wife swore it was coming from one side of the house when i unplugged the USB hub phone charger she never heard it again and we could plug / unplug it to reproduce the sound that magically teleported across the house.