I also have the hissing.. had it in my 06 and now in my o8... it only happens when I down shift. in my 06 it almost went away you can hardly hear it. In the 08 it is still loud!!! I had my local guy look at it and he could not figure it out so I took it to Altered Atmosphere and they could not figure it out eitherI did notice that when we put the headers on the 06 that the hissing was almost totaly gone so I am hoping that it will be the same with the 08!!! The guys at AAM and my local guy at the end both said it had something to do with the set up and the airflow. I will make a clip when I have time..next day or two..