The Spitfire does has a distinctive 'whistle' on approach, particularly the later Marks, (pre Griffon engine) such as the MkIX, but the Mustang certainly has 'that noise'. I also think it might be the air passing through the underbelly scoop, exiting through the aft doors. It could also possibly be the gun ports or barrels, but the latter would be 'plugged' anyway, preventing any significant airflow that could cause the 'whistle'.
Even though the supercharger would be inactive at air show altitudes, I have also heard that the 'whistle' was from the turbine blades; it will be interesting to confirm where it actually does originate.