Groung loops in computers

[Stormshadow]

I was wondering where to start looking with groung loops in a computer.

I have secondary PC AMD Phenom II which has been humming Via a USB Audio device via a mixing desk .
So it goes like this. Mixer---USB Audio---PC output---humming

At 1st i thought maybe i didnt ground the PC so i stripped it down regrounded and even used plastic stand offs too after rebuilding it back together ...nope the humming was still there.

I tested the same Mixer---USB Audio---PC output on my new AMD FX ,well no humming !!
So im stumped why it does this on one pc and not the other .
Any ideas people ??

[Tradesman]

First thought would be like you said a ground somewhere, next would be a device driver

[Stormshadow]

Hmmmm yes i was sceptical about it being the device driver to blame but the audio USB device is using the lastest driver .

[Stormshadow]

Then again if i didnt ground the pc it wouldnt work or would it ??

[Tradesman]

Still could, and an improper ground can cause feedback, also can check for other electronic devices or electrical lines nearby