I tried so many things to get rid of my noise.. and I finally killed it!
I run EPIA MII-12000 with M1-ATX. My initial setup was using the onboard sound, which gave me amazing amount of noise. It had killer amount of hiss/buzz/alternator/mouse noises.
I spent a while grounding things, and it did reduce the noise by a fair amount but I could not get rid of the mouse/hdd/cpu buzzing noise and alternator noise.
So I bought a power noise reducer and it did remove the alternator noise without hurting the sound but I still had this annoying buzzing noise.
Then I bought a Soundblaster Live External sound card hoping it might get rid of the noise but hell, the noise was still there.. but the difference in sound quality was well worth the investment so I dont complain. This however taught me that the noise was not from the quality of the soundcard but it was from power source/ground.
I read many different posts in this forum and found that most of the noise came from ground or -ve wires..
So this is what i tried.. cutting off the ground and -ve from USB cable. I first cut the ground but it didnt do a thing, so I cut the -5v wire and viola! the annoying buzz was gone! I do not understand how the soundblaster is still powered on without the negative wire.. I am guessing it might be grounding through the RCA cable instead of the negative wire in the USB cable. I did get rid of my noise and I am a happy man.
If you got USB soundcard and got unwanted noise, try cutting ground and negative wires from the USB cable! It fixed my noise!