I recently upgraded my computer and now there is noise coming from the speaker tweeters. The cable is connected to the A/V receiver using digital coaxial from my MSI motherboard. Everything else is the same. It makes even more noise every time I move the mouse also. I'm using a Crown XLS1500 amplifier which had noise before but this is much much worst.
Unfortunately, what you've written indicates that while you might have upgraded your computer's speed and graphics, you have managed to degrade its audio. You're hearing the crosstalk between the motherboard's digital and audio circuitry. If your computer's video card has an HDMI output and your receiver has an HDMI input, you might try using that instead.
Since your interconnect is digital, the noise must be traveling over the ground wire of S/PDIF cable. Do you have the option of using Toslink optical digital? That would break the electrical connection.