Knowing and Solving
What I take from the intricate details of the link from AMRadioLegend is that we can know a lot about RFI (Radio Frequency Interference) by reading this item, but that solving these problems is more complicated than simply knowing about them.
Some good reading there but here's the bottom line. You can't stop it.
It's on the A/C power supply and it's all around you in your house from the wiring around you.
I can go to a cottage out in the country by the lake....if there's power there will be noise to screw up AM radio. This is a modern day thing, never used to be this way. Before the mid eighties the power was clean. You could listen to a nice tube radio and it was clean....try it now. The noise doesn't originate in your house or come from a pole outside. Like I said, you can be at a cabin in the country, far from any major city, even in Canada, if you have wires to your place and power you will have the noise.
Here's how you solve it.....TURN OFF THE POWER and what do you know! GREAT AM like it used to be. Or go a distance away from the house.
And that's the biggest reason for the demise of AM radio, not the frequency response or it's not stereo or anything else...I used to go to a cottage at a resort every weekend and the daughter had a stable and some horses and had a radio in the barn and I took my station up with me(she loved it) and I asked her if I were to go onto AM would you still listen to me?, and she turned on the AM on the radio and the noise was end to end on the tuning dial....even out there, and she said how can I listen to this? No!....and she was old enough to have grown up with AM radio!
And she loved my station and had it on all the time.
Bottom line.....
I am fortunate to be in Canada where it's more friendly to hobby radio on FM as with AM no one is listening....why....because they can't. Even everyone here on this forum doing AM are getting a mile or whatever OUTSIDE to a car radio....not inside houses and buildings. Away from hydro(A/C power). Our hobby has to be listened to in one place as normally in a car you drive in and out of the signal coverage unless someone in the area sits in their car.
Life in the Shadow of the Poles
Over the years different noises have risen and disappeared on the MW dial.
Recently here inside the building I've noticed more noise than in the past and suspect Wi-Fi additions to neighboring installations.
We cannot ever know what the next door neighbor is doing, which makes new noises undefinable.
If street power poles are bringing noise we cannot sort it out in a useful way.
The MW band is a lost cause as it now stands.