Out of curiosity I finally cut open an OEM carbon fiber propeller shaft. If you have a shaft out of the car and flip it 180 degrees vertically back and forth, you will hear something slide inside and hit the bottom. If you don't hear anything right away, give if a few shakes and you will hear it. This sound is very distinct and sounds identical to the "rattle" noise so many have complained of. To my surprise it is just a piece of rolled cardboard! It's about half the length of the shaft.