So at the power entry, ground and neutral are bonded together. So if you are not grounded, then the neutral is not going to be very good. The last time I was ina building that had a hum like this, the ground was ok, but the neutral connection to the grid neutral was bad. So it stands to reason that if the neutral connection to the grid is ok, then your ground is bad.
Normally you go straight down to get a good ground. However many ground installations for things like radio towers will go down, but also fan out away from the tower. So if going down doesn't work, try trenching and burying as deep as possible a ground wire. A 6 gauge bare copper wire, burried in a 75' long trench is considered a reasonable lightning dissipation path for grounding, and these are run in multiple trenches radiating out from the point you are trying to establish a ground. If you can bury these at least a foot or two deep, hopefully in soil under w watered lawn, that would be ideal. For some critical tower locations, they would use water well drilling equipment to sink grounding electrodes. And for added grounding "power" the electrode might be hollow and filled with a rock-salt-like compound, and then put on a water drip system that periodically sends a couple galons of water down the pipe... with an access port at the top to pour fresh salt in from time to time.
The point is there are lots of ways good grounds are established, and saying the ground is too hard to do it right is a cop-out. Yes it can be difficult, but it is not impossible.
If you can pound a ground steak only 2 or 3 feet down, dig a trench, and plant a ground steak every foot, then run a heavy gauge bare copper wire in the trench, clamping it to every stake you planted... Cut the stakes off below the top of the trench, and then fill it all in with dirt. It's not as easy as a 8' ground rod sunk into dirt, but it isn't bad. Watering the lawn over the trench in a modest manner will improve the ground if it needs improvement.
Another common grounding technique is to attach to the steel rebar in the building's cement slab. The rebar in a poured slab is in quite good contact with ground, and makes a good source of grounding, as good as a water pipe. The slab holds some moisture, and thus the slab is as good a conductor as moist dirt.
I used to erect radio towers, and have seen a wide range of grounding systems used. I don't buy it that it is too hard to do. It may be too hard for someone being lazy, or careless... But it is hardly impossible. And as a DIY effort, just bury a 6AWG wire in a trench, and bind it to the outside of your electric meter of metalic conduit, and you should see a difference. Just be aware that as you connect the ground to the box, there could be some voltage potential there. It might not be much, but the energy from that hum is going to travel on that wire, so as you make the connection there will be some current flowing, so don't electrocute yourself in the process. You can measure the AC voltage between your new ground and the meter cabinet before you touch them together, in case you want to know how much potential there is there...