I had a (in hindsight) hilarious issue with this at one point.
I have a big GPU. Big GPU makes noise. That noise goes through the PCIe connector and into the motherboard. It then made it to the ethernet port. It went through my ethernet port to my ethernet switch, which then made it to the Raspberry Pi I was using for streaming, which went into my DAC, AMP and finally speakers.
It was because I was using shielded twisted pair, which connects the ground plane.
I have a big GPU. Big GPU makes noise. That noise goes through the PCIe connector and into the motherboard. It then made it to the ethernet port. It went through my ethernet port to my ethernet switch, which then made it to the Raspberry Pi I was using for streaming, which went into my DAC, AMP and finally speakers.
It was because I was using shielded twisted pair, which connects the ground plane.
Power isolation is an insidious problem!