I live in the UK and have heard the hum for some 8 or 9 yrs now. Mostly at night when it’s very quiet, as I suffer from tinnitus it’s actually a relief to hear another sound. It sounds like vary low rumbling in the distance, like you would hear from distant traffic or aircraft landings, sometimes like thunder rolling away in the distance far off. First time I heard it I opened the bedroom window to hear where it was coming from but it didn’t help, in fact it is better inside the house.
It’s with me every night and sometimes I concentrate on it to try to figure it out but mostly I just go to sleep. It doesn’t bother me, I just accept it as earths natural low frequency hum. Good to know I’m not alone though, now realise 2% of the population hear it. We moved house right out into the country 5 yrs ago and the hum came with me, albeit slightly louder and clearer now.