Then, perhaps, it is not the engine that makes the sound of which I speak. But when a plane enters into a dive there is an audible pitch that gets louder and higher in pitch until it is a scream. Is this air buffeting against the airframe? I thought it was the propeller turning faster that it is supposed to, or the engine being forced faster. Whatever it is, it is a clue as to your dive speed which is hard to guage on a flat screen.