Question
There is a fairly loud hum coming from our 50' x 40' elm. We can see bees throughout the tree buzzing around. We looked for a hive on the trunk, but didn't see one. Is this just pollination or what? Thanks.

Answer
I would guess what you are hearing is cicadas not bees. Cicada is an insect that makes a loud buzzing sounds as they mate and lay their eggs.

Cicadas are insects, best known for the sound made by the males of the species. The males make this sound by flexing their tymbals, which are drum-like organs found on their abdomens.

Cicadas belong to the order Hemiptera, suborder Homoptera and family Cicadidae. Leafhoppers, spittle bugs and jumping plant lice are close relatives of the cicada. Hemiptera are different from other insects in that both the nymph and adult forms have a beak, which they use to suck fluids called xylem from plants.
Adult cicadas, also called imagoes, spend their time in trees looking for a mate. Males sing, females respond, mating begins, and the cycle of life begins again.
Here is what they sound like

http://www.cicadamania.com/images/sounds/princeton2004.mp3