I sometimes get a hissing sound that moves around the various speakers of the car. The hissing comes and goes when ever I hit a bump. I can also make the hissing go away by using one of my power accessories (power seat, window). I have returned to the original installer (Circuit City), and they checked the ground on the unit and said it was fine. Funny thing is, whenever they open the panel to access the unit the hissing disappears for one or two months and then reappears. The unit is bypassed (using ducatiboy's instructions) and I also have a rear DVD player, in my 2006 Impala SS. It is really annoying. Any suggestions?

yeah. dont go to circuit city for installs