Heat kicked on

Or the refrigerator.
I live in the middle of the Silicon Valley, one of the electrically noisiest places on the planet. I have a long list of solutions for this issue, not one completely effective. I use them in combination to get the job done: star-quad mic and instrument cables, ferrite beads for USB cables, iron rings, balanced step-up/step-down transformers… All contribute to the solution. My studio is dead quiet now as far as electrical noises are concerned.
The exception is star-quad power cables—no difference at all but not very expensive because I made my own.
We had one case where an AM radio station down the street caused it. Solution on that one: move.
My San Jose house had a radio tower 8 blocks away. Normally, not an issue unless I used an effects box on my guitar — useless! For the last 21 years, I’ve been renting that house to San Jose State music students and no one has complained about this, no idea why.