does the sound happens on xp?
my netbook has all the contrary, when on xp and no sound is being played i hear a noise all the time, clics and pops and hiss all the time, in pclinux that doesn't happen
possible reasons n your case for this behavior is wrong sound driver being used, also volume levels on unused channels too high(disable the channels you don't use and/or lower those channels)
also the mouse could be doing interference but that is unlikely since it only happens when the sound is being played
sometimes when you overclock a machine this happens but i doubt that you can overclock that laptop