The whistling is caused by pursing the lips and blowing gently, varying the pressure and aperture of the lips to alter the pitch of the sound emitted. It can be almost musical (LIKE BAGPIPES

) if done correctly.
But more seriously , the A post finisher (outer black shiny thing) sometimes doesnt quite sit down right, allowing air at speed to get under and thru to the door frame, and lo there became a fearful wailing.
My Pepper white
MC, and the Yellow/Black
MC both had a variance. Some goo strategically placed under offending finisher does the trick. I hoped that 'recent' cars would be sorted, words will be had in certain ears.
NO Whistling Allowed, nuff said?