Glad to hear that it all turned out well. It's also good that it's in the archives for the "next guy."
My I350U has the exact same howl you described. I've just left the PTO engaged, but I've been a little concerned about safety. It's good to know that the howl is normal so I can leave it disengaged instead.
As for seasonal disconnects, my I300U has one but my I350U doesn't. Maybe it's a model-specific thing; I don't know.
Mark W. in MI