From the ALSA wiki
I have recently bought a Toshiba Satellite laptop which was equipped with that sound chip. The problem I have encountered was that while under Windows the sound quality was good, under Linux (Fedora Core 6) the sound was very noisy and there were a lot of cracks. Southern_Gentlem from #fedora channel has suggested that the noises are caused by some feedback from line-in and that they can be sent into oblivion by muting that item. As the laptop does not even have a line-in socket, I have muted that without hesitation, and voila, the noises were gone. Hopefully this text will save someone from intense head scratching.