USB Cable interference problem

[Starmonger]

Please bare with me whilst i try and explain the problem: First off, I was in cubase using the usb cable and VC- i get a slight interference sound which is only noticable when you compress a sound or make it louder. I only started to notice this recently as it is not noticable until you do what i said, but i guess it has allways been there. so anyway I went and bought a cable which would allow me to still use usb for the vc sync but then send the audio out via the main output on the synth. i thought this would solve the problem.
So i have my usb cable plugged into the synth and computer, and my audio cable from synth to computer, however now i notice that whilst the usb cable is plugged into the synth, if i move the mouse i can hear a squeeking sound(yes haha), and otherwise there is a consistent crackle/interference anyway. but when i pull out the usb cable this goes. all this is without cubase or vc and even with the virus turned off. So bascily there is some sort of shielding problem or grounding problem to do with the usb cable which is feeding static/interference through the synth. i have tried all the motherboards usb ports and still the same thing. Please anyone know what can solve this? atm i am screwed and cant record anything due to this. Im guessing its either a problem with my computer, or the usb cable rather than the synth at fault.

I forgot to mention, the cable i bought is an unbalanced dual big jack to 2 phono style cable.