I have been fighting with my PowerMac and Laptop for a long while with audio crackling and hum that gets worse when you move the mouse.

I have tried different speakers with the same results.

My speakers have both 3.5mm and RCA inputs.

The noises only happen when both computers are connected. If either one is by themselves, there is no noises. If I swap inputs between the laptop and PowerMac, the results are the same.

Everything is plugged into my UPS.

Here is what I tried so far to fix the problem.:
1. Turned off all sounds in windows.
2. Attached ferrite beads to the audio and video cables.
3. Bought higher quality cables and balanced ones where I could.

Everything has made no difference.

Any ideas?