I have another thread where I complained about the same. Open the window all the way, which allows it to flex a lot more. Take a 2 inch x 1 inch piece of felt and stick it on the upper right part of the window that goes into the window track, and the same on the upper left. The upper left part requires a bit more force as its still in the window track, but the upper right part moves quite a bit when open and easy. Have someone close the window while you are holding the bottom of the felt and it will seal tight into the track and fix the rattle. Mine was severe even at lower volumes with any type of music, now its silent.
This works great from Amazon
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