The buzzing sounds made by bees, flies, wasps, and other insects come from the rapid beating of their tiny wings. Some bees beat their wings up to 230 times per second, creating a loud enough sound.
In addition, some bees, especially bumblebees, are able to produce a specific buzzing sound by vibrating the muscles in their thorax during flight. This “function” is needed to pollinate “finicky” plants that release pollen only at a certain frequency of vibration. This is characteristic of tomatoes, blueberries and some other representatives of flora.