Charles, Generally speaking, a loud buzz from a electrical
device like that means the metal plates in the transformer have de-laminated. This is really common with the transformers in fluorescent overhead lights
. You don't usually hear the 60 Hz hum but the 2nd or 3rd harmonics--120 Hz & 180 Hz. It's not a danger or anything but quite annoying, as you probably know.
The fact that it only happens with two lights makes me wonder though. Can you get down next to it and try to determine where it's coming from? If you're going to poke around, make sure you wear a rubber kitchen glove. If it's the transformer, there's no cure except replacing it or the converter. Getting parts could be a problem. I guess if you bought a 2005 FT and the worse you have to do is replace that, then you've gotten off pretty well.