Greetings:
Not sure if anyone mentioned this: Have you checked each mixer channel
for noise ? I've found that the CD channel and another channel (AC97?)
on my SBLive mixer produce an incredible amount of noise that disappears
entirely when those channels are muted. I don't know what channels you
have available on your audio chipset's mixer, but you should check each
one for noise.
CD channels seem to always produce much noise, probably due to the cheap
drives I buy.
Best,
dp