When I first heard the sound, I thought it was the kids across the street playing basketball. I heard it again and didn't pay attention to it. Then I noticed that I hear it every time I shut down the engine. The sound is exactly what it would sound like if the windows are up engine off and someone bouncing a basketball about 30 feet away from the car. It stops as soon as I open the door and all extra power turns off. Is this normal? In case it helps, I have the upgrade sound group II with the more powerful amp. Could it be the subwoofer powering down? Thanks for any help or insight.