Damn! I was going to post that it's a pebble in the brake. Why would I know this? Because I had exactly that happen to my P3D twice, and someone else posted the cause of the issue after experiencing the same. Apparently, the other person was told by Tesla that there's a cover that they can install to prevent a reoccurrence. I've been lucky and haven't had it happen again, but definitely know that feeling of thinking a metal-on-metal sound is not a nice thing to hear from your car.