Interesting case, I doubt that two units would have the same flaw out of the box. Or if it was a production error, there would have been more reports.
Sure you are not suffering from Tinnitus? Just kidding!
Since it apparently makes the noise when nothing is connected, have you tried plugging it in at various outlets (different rooms and such), perhaps there is something wrong with the power at your place?
If it’s not in the unit (or the way it is hooked up), then you have to start looking at other things outside the unit. Or at least rule those out before deciding that yes, it is in the actual unit and clearly in more new units.