After leaving the computer on for a while I reproduced the problem on 5.10.0-6 as well, even after disconnecting the SPDIF cable.
Considering that I've been using this machine for at least one year and I've never encountered the issue, I believe the relay in the amplifier is going bad and pushing noise into mains. I would expect the power supply to filter it out though, as it's a quality one: Sharkoon SilentStorm Cool Zero 850W. Out of curiosity, were there perhaps changes related to power management in the platform or amdgpu drivers?
