That kind of buzzing is usually from the Composite Video leaking into the audio wires. AV cables are using coax and shouldn’t be subject to it, it must come from the console itself.
Are you willing to experiment?
What I would do is nip the composite signal as soon as it comes out of the chip. Looking online, it should be pin15 of the chip S-RGB A BA6596E. make sure it’s the correct one by testing the continuity, and lift the pin or cut the trace next to it. Or maybe there’s a jumper not far away. If you still have interferences, then I’m at a loss as to what could cause it.
If you want to keep a Composite out, you could restore it by having a wire from the pin to the connector. A simple wire should be enough, but you could be extra careful and use a bit of coax.