Originally Posted by
dabac
What you're seeing is brake rub from the frame/fork flexing. In an equally flexy frame/fork using rim brakes, you'd seen the same with the brakes set up tight.
....Annoying as it might be, the good news is that brake rub from frame/fork flex doesn't cause enough drag to mean anything for your overall speed.
+1 on both counts.
Fork flex allows the hub to twist slightly causing disc rub. if it only happens on one side (ie. right turns only), you can sometimes bias the brake to the other side a bit. But it's not worth opening it enough that braking suffers.
The incidental contact rubbing doesn't have any real force behind it so there's no meaningful friction involved. Car brakes often exhibit the same noise, and you can hear them chirping. If it were an issue the brakes would heat up and get cooked in short order.