Wow... just to say we ARE reading your post but I doubt any of us know what to do about it. The first thing I would do is to see if I could borrow a comparable (ie reasonably good) receiver for an hour or two, plug it in and confirm that the ringing is not there as well.
If your receiver makes a noise that other receivers do not that should give you a whole lot more credibility when speaking with mfg or dealer -- otherwise they're just gonna tell you to put the tinfoil hat back on and call them if the problem keeps happening
Please don't interpret my last comment as doubt about the problem you are hearing (my subwoofer is making noises that sound uncannily like a fireplace chimney with a vicious storm blowing overhead, even with the input cable unplugged), just a warning that if you are dealing with the drones in the audio world they're going to be pretty sceptical so you had better be well armed.
Armed with information that is. Don't look at the avatar.
