Heard many theories including air conditioners but they don’t add up. I don’t know all the causes only how to counteract enough of the side effects to keep systems enjoyable.
I wonder if it has anything to do with the moisture in the ground and grounding conditions for electricity. It just recently rained a lot here. I know a guy in the Midwest who pours water on his ground rods during dry spells to improve the sound of his stereo system.