When something like this begins suddenly, as I assume it did two years ago, I first think of defects in the electrical distribution system. The daytime aspect may involved electrical demand. Demand typically is rising sharply by 8 a.m. and has fallen back to the 8 a.m. level by about 5 p.m. Which makes me think of a defect that produces an arc under the heavier load. I'm not sure about he seasonal aspect. Your month-to-month precipitation averages are remarkably similar through the year, but other things can be involved, like processes that eject particulates during some seasons and the intensity of rainfall events in some seasons. I once had a recurring problem caused by lint from a cotton gin that accumulated and formed arc paths across insulators until a particularly heavy rain or a series of more gentle rains washed it away, and it didn't return until the next ginning season. It was a problem across a wider range of frequencies, but it originated within 100 feet of my antenna. Yours may be radiated by a floating segment of wire that happens to be resonant at about 28 mHz. That can be anything, including a broken ground lead on a pole. Sounds like it's time to mount a hunt.
Of course, there can be other odd combinations of factors. Daytime only might suggest a solar charger or associated inverter, with temperature or pollution effects providing the seasonal aspect. Come to think of it, had I been close to the cotton gin itself and it happened to have a noise source in the equipment, I would have heard it only during the ginning season, it would have been daytime only, and it might well have decreased over time, since there's a lot of throughput early on and less as time goes by. Any kind of seasonal activity like that in the area?