This can be a real bugger to track down.
If you have isolated everything in the house that could cause it, it may be coming in on the mains feed to the house, or being transmitted by arcing in the power grid.
If you want to check if its a bad connection or a cracked power line insulator, the time honored way (Back into the '20's even!)was to take a portable radio tuned to the worst of the noise, and a sledge hammer and go down the street and whack every wooden power pole at the base with the sledge. If the noise suddenly increases or disappears, you likely found the problem pole.
Take the pole number & call the power co. to come repair it.
I had a very bad noise problem and used this method to check the poles on my street. No change.
I called the power co. & finally, with the help of the English language, got put through to the Chief Engineer.
We had a fantastic talk, and finding I was a Ham, and worked in electronics repair. I told him what I had done to try to trace the problem, he said he would look into it.
By golly! 2 weeks later it was noise free reception. A week later he called me back to make sure the problem was solved.
He found a bad ground crimp in the power grid grounding system across the freeway from my house. (About .6 mile).
"Noise free at last!"
BTW- I tried a microcomputer controlled security system and it killed all reception with a 20DB!!! noise level. I went back to the old relay system & problem gone.