If its as soon as powered before the OS boots etc then I'd say the amplifier chip on the integrated sound has noise maybe from a bad capacitor in the amplifier circuit. If this is the case then there is no volume control to make it any quieter its stuck with this level of sound. Only way around this would be to disconnect the speakers internally if its an annoyance to the point that the hissing has to stop, or replacement of the main board which is costly. How is the sound quality when playing music etc, does this HISS wash out the audio? They make USB Audio adapters which can correct for sound problems but they dont pass the sound over to internal speakers. They work with headphones or sound systems connected via a stereo mini jack.
