[16newcamaro]

Finally found what was causing a hard plastic knocking noise ( sounds like two pieces of hard plastic hitting each other ) when the window was down especially when going 65+. Someone said this is common on all cars, which might be true however not in this case. It turns out the left and right trim pieces which run from top to bottom along the rear window were not locked into place properly, especially at the bottom. I had the dealer repair them and guess what, no more noise.