I suspect the reson the EMI doesnt get pulled down to ground in the analog section is that it may have already been ‘encoded’ for lack of a better term into the audio signal by that point somewhere in the audio processing stage on the AVR, its no longer electrical interference but artifacts and noise in the audio signal itself.
I think you may be right about checking for a ground differential, partically between the PC and the AVR. @sb-woo I would start here in diagnosis and for now focus on the PC and the AVR in your diagnostics. I would guess the noise is already part of the audio signal before it passes through your RCA cables. Keep in mind during your troubleshooting ferrite rings can be used on the HDMI cable too, not just analog audio signals