Wonder if you are at one of the main feeds for the water main that is out burried under the street at your house. Supplying water for lots of other people beyond you too.
That water flowing through the main could be resonating up from the street through your water feed and by conducting onto your sewer line up into your house to your cement slab, then on up through the walls of your house like the bell on a trumpet putting out the hum sound.
As people come home from work in the evening the more water they use, taking showers, washing clothes, flushing toilets, washing dishes, cooking could be increasing the resonating tone of that water main, then conducting it up to your house.
Have to do the three foot 3/8" extension test on your water pipes like a stethoscope to see if you can hear any trace of the hum coming through your pipes.
Louder hum at night that during the day.
Hmmm . . . .
