I have the same problem in my 2007. Same model.
I've noticed that switching the 15 amp breaker off and back on temporarily fixes the problem. But the noise comes back when I turn my 12v light off and later turn it back on.
I've also noticed if I leave something 12v on, like the vent fan, I can turn other 12v things on and off (like lights) and I don't get the sound.
It's only when all 12v accessories are off for a bit and then I try to turn one on that the sound comes back.
I do know my battery needs to be replaced, but I wonder if the converter is bad too.