try to scrape that scale off. if it stops whistling, that may be the reason.

Like other kettle, there is an effect called "impinging shear flow instability," and it is the same reason that a vacuum cleaner hose will make a sound if you whirl it around your head. The vortices cause pockets of oscillating air pressure or a vibration in the air that travels through the intervening space and strikes your ear drum as sound waves.