"1. Sound does not happen when running it in reverse."
Ah. That's a good sign, actually. The motors move a bit back and forth depending upon their direction, so, a loose truck, loose motor mount screws, or just something barely touching (direction-dependent) a flywheel.
Try putting some mileage on it in reverse; the piece may move out of the way with time. Turn the engine over and over in your hands several times (very high tech).