Bumblebees and some solitary bees latch onto blossoms and move their flight muscles rapidly. The high-pitched buzz shakes recalcitrant pollen grains from the anthers of plants onto the bee's head. Honey bees don't normally do this, and are less efficient pollinators of certain types of plants, including members of the Ericaceae such as blueberries, cranberries, and Pieris.
I went back outside to listen more closely. Yup, bee sticks face into flower, makes a short, sharp buzz like one of those pneumatic wheel-remover things used during auto-racing pit-stops, then moves to the next flower.
Buzz Pollination, bee astronaut.