Stick your ear next to one of the front speakers with ignition on, headunit off......
You have BOSE? If so, it's called floor noise and is due to the amp turning itself on whenever the ignition is on, even if the head unit isn't on. All of them do it, and it depends how good your hearing is as to whether you pick it up. It bugs the heck out of me but I want to have the install done properly so have an induction kit to keep me smiling!
I think this is something to do with the hands free car kit isn't it? That's why the amp is always powered up. The hissing is just white noise generated by the amp. Not sure if there's anything that can be done to solve it, apart from playing a CD that is...
May well be, but either way it is always on. Highlights two things - poor quality amp and poor quality grounding.....
It is still there when playing the CD, it just gets interrupted by the music.