If the CPU noise is still an issue, try sending the motherboard audio into tip/ring (XLR pins 2+3). Leave the shield open on one end or the other. If the noise is eliminated but returns after a quiet period it means your sound card is going into low power standby and its outputs start floating - you'll need to prevent this.
I had noise for years across two houses, two different power supplies, two different motherboards, and two different powered monitors. Signal into tip/ring and open shield finally fixed it. The wiring suggested above didn't work for me.