Why Do Bees Buzz While Flying?

We can hear buzzing sound when bees fly. But when they land on flowers, the buzzing disappears. What is the matter? Bees beat their fan-like wings to circulate the air with a speed of 200 times in the twinkling of an eye, so we can hear the buzzing. When staying on flowers, they stop beating their wings, so we cannot hear the buzzing sound.