Separate the cables inside the computer, usually this happens when the drive or audio interface cables are tied to the data buss cables or the power supply wires are tied to data or audio cables.
Inside the computer case, Separate the audio wiring from the data buss cables, as far as possible...
There will be a cable inside too, for the usb jacks on the front panel of the computer, separate if these are tied in with other wiring, or the audio wiring.
Experimenting with separating the wires inside, changing the position of the internal wires, cut the wire ties, so that wires and cables are no longer bunched tightly together.
You should be able to find the group of tied wires that is causing it, and separate them, with distance between the cables...
2. Unplug your internet connection (ethernet or whatever) and see if the noise goes away.
3. Find the computer audio mixer
click the speaker icon in the lower right hand of your screen (or double click it)
click on mixer
lower the mixer input volume one at a time, until you find the input with the noise.
All PC systems have this mixer, you just gotta find it...