Bose or Sound Package Plus?

If SPP (and perhaps Bose, but not sure - I expect it to be the same), the amp is on a separate circuit and may be timing out longer than the head unit. As is the case with many, many (dare I say most?) amps there is a bit of hiss at high volume settings which is simply analog noise in the system being amplified. It's there as long as power is applied to the amp. With the system on but no audio source (CD stopped? I'd suggest using the TP button which serves to mute the audio but I don't know just how and where in the system that's wired in.) can you change the volume of the hiss by changing the volume setting on your head unit? If so, that's probably it.