It cannot be the backing plate. I know this because I did a test where I barely slid the rotor in and spinned it. I still got noise. And the scraping is not constant. It's almost as if the rotor is warped from the inside. Now what is also interesting is that if I push the rotor in with force, hold it in place, and spin it, the noise goes away. So I tried tightening the lug nuts on the wheel to make sure it presses the rotor as far back as possible and it still makes noise.