Is this a separate sound card or an onboard one?
If onboard it is likely to cause noise as and when you access the PC. The amplifier within the mixer will also be picking up on this and thats why it will be more apparent.
Also check you do not have your audio cables touching power cables. This can also lead to more noise.
I used to have this problem with noise and got a seperate PCI soundcard which solved it.
I use this one:
http://www.dolphinmusic.co.uk/produc...hz-24-bit.html