I'm not Kreskin, but......I'm going to blame the 'NOISE" on improper
shielding of computers and/or SPEAKERS. If you are getting 'noise" thru the speakers -and attributing it to the computer -
try moving the speakers farther away from the monitor, and tower....Have you tried different speakers to see if the same "noise" occurs? Tried a
diff sound card?
or even a small thing like "tweaking" your sound settings for the Scard...or
muting things such as the PC beep??
Also there is a thing called:
SPREAD SPECTRUM in some bios's...
that can help with computer generated noise if :
'ENABLED'. [Yea, some decrease in performance BS - from OUTDATED info.] This sometimes helps with Radio Interference....

ATI or Nvidia makes no difference , as long as it works properly with the drivers PCLOS has detected. If you are a HIGH END GAMER, then you should be using a windows product designed specifically for that O/S, to achieve optimum performance.
Some of my LINUX friends have had good luck with a reasonably priced
fanless ATI Radeon HD 4350 ...
The exact MB/ model is your preference -512 should suffice....
READ THIS NEWS ABOUT AMD/ATI and OPEN SOURCE... Some here might be interested also.
> IMPORTANT - ATI Video INFO :
HERE ARE THE ATI LINUX DRIVER RELEASE NOTES (pdf)....
Power supply fans, CPU fans, and video card fans
can be culprits - but the NOISE must be ISOLATED to solve this RIDDLE of a post.

Do you have a WINDOWS o/s on your HD (dual boot) -if so does it do it with that.....or does it occur with a LIVE CD also

JN.