So I listened to the noise, which I wouldn't call grinding. If your sound is the same, it sure sounds like chain rub because it is intermittent with the pedal stroke.

And since it only happens when the frame, wheels are under lateral stress, it could be anything that might flex enough to cause a slight rub. It almost has that fender rattle, but you would hear that if you went over rough pavement. Plus I have no clue if your bike even has fenders.

John