Does the original OP live in town. If he lives in town, connected to a city water system and a city sewage system, then yes, it is most likely sounds and vibrations are coming from the utilities systems.
We lived in a small town many years ago. The house we lived in had city water and sewage. Every time someone on our side of the street flushed their toilet, the water in the bowl of our toilet would raise and lower. It was weird seeing that, but we finally accepted that.
The building I work at is a huge building. It's almost 1/3 of a mile long and had 5 floors with several wings. When someone flushes the bathroom toilet, pretty much any floor on the wing you are in, you can hear the water rushing through the toilet and in the drains in the floor.
So, if connected to city services at your home, the sound absolutely does vibrate through water and sewage lines.
If the OP lives in the country, on a well, has a septic tank with their own finger system, has an all electric house, then the "hum" is definitely coming from a strange source.
For the OP of this thread, I agree, Captca is getting worse again. I bumped into it several times again yesterday.