My Packard developed the same whistle. Carb cleaner didn't show a leak either.
I took the carb off and snugged up all the screws holding it together (the center section attaches to the base with screws from the bottom so it can't be done on the car).
That cured it right up. The leak wasn't enough to make the car run rough, just enough to make an annoying whistle.
I told people it was the supercharger winding up.
