An account in Canada, Dartmouth B2V1P8

It sounds like someone is sanding wood with an electric sander in a different room than I am standing, say 20 m away if it was really a sander on the other side of the wall. Or like a fridge with a bad compressor leaning against a wall, or like a ceiling fan, or a distant fog horn that continues irregularly. None of those exist or are on in my house. brrmmmmmm brmmmmmmmmmmmmmmm brm brm brmmmmmmmm... It is only on the upper floor of my house, but in all rooms when I walk in them. It is a little louder in a few places (near corners of rooms). No other family member can hear it. I went outside to see if neighbours were doing something in a shed or garage, but nothing. I can't hear it outside. Checked the attic, and lower floors but can't hear it in any of those places. Turn on and off fans, electrical equipment, anything we could think of that could cause a low level vibration. 9:30 pm Atlantic Time March 3 2021, -2°C, clear sky, 21km/h with gusts to 41km/h. (note your date menu ends at 2020) My house is about 2 km NW of CFB 12 Wing Shearwater runway.