That's crasy, after every system sound or music, if my mac keeps silent in about 25 seconds, there will a great noise from the audio output. seems the sound card will be turned off if there is no more sound in about half minute, there is an electricity shock duiring the sound card turns off. i tested on one old model 990, almost the same. does anyone else have this problem also? is there any fix for this? i mean i do not need mac os turn off my sound card to save the power, i just don't want the noise.