Only just seen @123toid tagged me in this, been busy with work recently
If im understanding your explanation correctly I have the exact same noise with my studio monitors and my RTX 3090 workstation although for me I can only hear it if I put my ear right up to the tweeter so I can mostly ignore it. My understanding is that @elliottdesigns is correct about it being coil whine however im dont think he is right about the power supply being part of the problem, its likely only coming from GPU. I have a very high end 1200W platinum rated PSU from EVGA and I have noise.
In electronic components such as graphics card you get coil whine when power runs through certain PCB components generating electro magntic forces. The strength of these forces are dependent on the quality of the PCB design and how much effort has been put in to mitigating them. These EM forces in turn cause the PCB component to vibrate at an audible frequency. However the generated EM forces dont just make anoying noises they can also introduce interference noise in electrical signals (EMI) , particulaly low voltage circuits e.g. a modern nvidia GPU core runs in the 0.9-1.1V range, hdmi signal pins are around 0.5V. As the GPU core frequency changes with PC load the ammount of power running through the card also fluctuates. This is why the noise changes as the PC does things. IIRC RTX 3080 and 3090 gpus can pull well over 300A across the core under max load and nvidias lazy design specs let the card have momentary spikes in the microsecond range at 1.5x or more their rated TDP, these large current draws can put out massive ammounts of EMI (fun fact thats why front panel audio IO on a PC case is bad to use, inside the case is a literal mine field of EMI)
These interference noises can travel out from the originating component along any conductive path. For me they seem to travel back through the motherboard and out down the USB cable that connects my audio interface to the PC. If you have your PC connected to the reciever via HDMI that is most probably how the noise is getting into your audio system. Even if the reciever is not set to your PC hdmi input or is even off simply being electrically conected is enough for the noise to make its way to the audio system
Unfortuntely, unlike ground hum, this type of noise is unlikely to be resolved by isolating the power to certain parts of your audio system as is trasmitted via an electricaly conductive signal connection not via the power lines. The one exception to this statement is if your building wiring is not properly earthed, In that scenario the EMI noise could be transmitted through power line. If that is the case you have a more urgent issue than noise in a speaker and should get an electrician to resolve it ASAP!
