ok, ive always run computer audio through either an HT amp using a line in or through a powered sub/sat external speakers setup and have never had a problem. headphones also dont reveal the noise.

ive just upgraded to some EV active PA speakers and when any desktop PC audio line out there is a very noticeable constant electrical noise/hum/static. I've read some onboard audio chipsets are noticeably noisy, so have tried an Xitel XFI External USb sound card and still get the same thing... 

using the lineout on a laptop/iphone/ipod or the xitel on a laptop all makes the noise go away and performs perfectly...

my workstation is an HP XW4400 QX9500, ive tried a couple of HP Corporate Desktops (Elite 8300 i5 and 6200 i5) and the same thing, my macbook and 8470p Elitebook all work perfectly.

Id hoped an external sound card would do the trick but now im stumped.

can anyone point me in the right direction?