@MinusKelvin Thx for the input.
I was finally able to solve the today, and it was crazy: for some reason when the GPU is understress it uses the cables to the speakers/monitors almost as an antenna.
When replacing the cables with shielded 1/4 inch cables instead (shielded have a “bandage” of copper around the entire wire to shield it from interference) the problem went away completely.
Any normal guitar / 1/4 inch cable will cause the interference to come back.
Really, really interesting. But in my case, it’s fixed now.
Hope this will help someone!