Yeah man and it is confirmed. New buzz this morning. Unplugged hdmi arc and buzz stops.
This is really interesting. RF interference is a very common problem in audio systems. However your case is a very unusal one.
What is happening is that the TV, and or HDMI cable, are picking up the signal radiated from the superhet oscillator in your AM radio that is mixed with the incoming radio signal.
It is most likely the TV chassis and the HDMI cable ground that is pick up this signal and feeding it to your receiver.
Now the semiconductor junctions in the ICs, in the early high gain stages, can be excellent demodulators of radio frequencies. This is just like the crystal radio sets of old. So your high frequency signal becomes rectified (the crystal radio affect) and you get to hear an audible buzz.
Your solution may be to use a hybrid optical cable with voltage inserter to isolate your HDMI cable from the receiver as much as possible. However this may well not work as the audio link is via copper.
The real solution is RF traps at the early voltage amplification stages. However with modern circuit boards this would be next to impossible.
However I fail to see why this is a problem, as if you are listening to your AM radio, I would logically assume you are not listening to your receiver, and you can turn the latter off.
