Thanks for the added info about using lower volume from the pc but turning up the Audionet receiver; by reducing the desired signal strength of the pc, you've been unintentionally making it closer in volume level to the unwanted noise caused by interference. Since you say that the noise disappears when the cable is unplugged from the pc, my gut says that the interference/noise is coming from the pc/power supply as mattkhan said. If the interference were cell traffic like the HiFi shop mentioned, I'd expect that the noise would be the same or even worse when disconnected from the pc. Note that unless the pc power supply switch is turned off or the power cord is unplugged, there is still power from the power supply to the motherboard etc. which explains why you're still hearing the noise when the pc is turned off (but not powered off completely or the cable is disconnected).
My first suggestion is to do what you are now doing-keeping the wanted signal "hot" or as strong as you can safely do so that minimal amplification by the Audionet is required which would reveal the noise. This by the way is why devices signal to noise ratio (S/N) is always calculated at full volume.
My second suggestion if this is insufficient to reduce/eliminate the noise is to invest in a USB audio isolation transformer (as recommended to me earlier by power user eve) between the pc and the Audionet receiver. This would require one USB cable from the pc to the transformer and a second USB cable from the transformer and the Audionet receiver.
While I haven't yet bought one because I have minimal noise coming through my system, Topping Audio out of China has two such transformers the HS01 and the HS02 (the one that I might yet buy) at
https://www.toppingaudio.com/products-list/others and available for sale through Apos Audio, Amazon, etc.
There may be better transformers out there but I think that this purchase would be worth considering IF you are still hearing the interference/noise.
Good luck!