Some of the buzzing is probably caused by the movement of the wings, as explained by Shadow, but they also buzz when the wings are not in motion, so it is probably the vibration of the muscles in the thorax causing the thorax to vibrate that makes the buzzing sound...
Thankyou Clanad I feel I have learned from this, it also seems that your are 100% correct, I checked again and Scientific American says that some species are capable of moving the wing and thorax muscles to shake pollen from flowers. Another source for the famous sound.