When using my standard iPod headphones with my macbook I can hear a hiss in the audio signal. It is the same level regardless of volume, but it is to the point where I can tell whether the audio port has been shut on or off. For example, if there is no sound playing, there will be no hiss whatsoever. Then if I adjust my volume controls for example, the audio port pops on, I hear the blip sound of the volume control, and then I can hear hiss for about 5 seconds in the background after which the port shuts itself back off (presumably to conserve power). Funny thing is that I just got a new logic board last week and this issue was present both before and after. So is it normal, or does replacing the MLB not replace the integrated audio chip somehow.