Has anyone ever heard a "knock" on your rear windshield about 15 seconds after you turn on your car? I hear it almost everytime i start my car and my friends always freak out cuz they think someone knocked on the window. Another thing I noticed is that my factory stereo seems to just cut off while i'm blasting my CD. I turn the knob to the second to last bar when i blast music, and sometimes the CD will just stop and i've even heard screaching sounds out of the speakers when i turn it off. I'm thinking the stereo has to do with it overheating or too much wattage, but the knock on the windshield is just plain weird.