It could be floating mains on the TV trying to find its way to earth via the Atom. You can’t safely lift the mains earth on the Atom but you could try the signal earth switch and see if that makes any difference.
If no joy then you may well be better off using the TOSLINK optical connection instead.
Thanks, I ended up getting an optical cable and that worked out.
The dealer also helped me out, so apparently there are extremely many options hidden in the Sony Bravia that switching to PCM stereo and turning off all the Dolby stuff and having HDMI arc output isn’t enough. Too complicated to explain but the HDMI also ended up working anyway!