I've been hearing this since I was a kid.

It used to occur late at night, exclusively.

This noise used to thum in a slow deep rhythm back in the 90's... recently it's become loud enough so that more people are noticing it, the rhythm of it has also quickened considerably.

These days, it even continues on throughout the day, some people I know have even started to notice it.

I never really thought much of it, but recently it's become almost unbearable, so that I have to distract myself by turning on background noises at night.

When I do manage to fall asleep without any background distractions, I am sometimes awakened by strange, almost mechanical sounds, that seem to disappear as soon as I manage to come to my full senses.

I've always had a strange feeling about this, mild apprehension, it used to scare me as a kid whenever I'd hear it approaching.