AnomalySounds like a high idling engine outside (i.e. the kind of frequency you'd get for a cold start ~1500rpm give or take for a 4 cylinder engine whatever that works out to be) Other than, it was initially very frustrating, but I've learned to live with it as a feature and don't let it bother me when it occurs
UK, Towcester · 2019-12-26 · from the WORLDHUMMAP archive
An account in UK, Towcester
What they reported
First noticed: 2009
Pitch: About 140 (hum not present when I did the survey)
Loudest: During the night
Where: Equal indoors and outdoors
Changes with weather: Seems to be much more prevalent in colder weather (so far as before reading much about it, I wondered if the source was something temperature related that was being switched on)
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This is a Hum report from Dr Glen MacPherson's World Hum Map, an open, worldwide record of people who hear an unexplained low-frequency hum. Reproduced with permission and with thanks; the project is at thehum.info. We remove anything on request. Location shown is approximate.