Is the sound coming from your speakers or your car? Might just be the aerodynamics of the car or a bad weatherstrip (those rubber seals on the insides of your doors). Check those out and see if they are in good shape. Also I noticed on my car....whenever I changed my wiper blades to rain-X ones, they have holes all through them and it was causing it to whistle when the air was going through them at high speeds. Just a suggestion.