That hum

A fair proportion of people say they can hear a very low frequency audio hum. I can hear it. I have heard it from very early morning (about 5.30am UK time) until now. At first I thought it was someone's washing machine, or perhaps a plane. It sounds a bit like a distant washing machine.

Could it be the distant noise of traffic? Low frequencies may propagate through rocks. It is a very low level, but definitely there. It is worse today than I recall. Weather conditions are a little misty and we can sometimes hear traffic on the A14 road (about 2 miles away) clearly on misty days, which must allow audio signals to propagate further. Normally we do not hear A14 traffic.

I expect someone could "see it" with suitable audio transducers and software.