Can't explain the time thing. It was during a real cold spell and it is a intelligent charger so my guess is it wasn't reacting to the light of day, but the temperature of the day. The colder it got, the lower the voltage the charger would charge to prevent over charging so it would quit at night.
I had a spare FT240 43 core so I wrapped the a/c wire 9 times through the core and guess what? No noise. But then again it is much warmer now so its charge profile changed too.
I own a ProMariner 35 amp 3 bank charger for my 10 AGM battery backup and it nary makes a peep of RFI with no external mods. I guess the difference is in the price. My charger was $400 and that rinkydink noisemaker was maybe $100.