Swarm of what?

While sitting outside this afternoon my attention was caught by an intense buzzing sound. Looking up I saw a cloud around 50 feet diameter and twenty feet off the ground move close to our house then drift off over the lawn then it moved off into the distance across a field. The cloud must have consisted of thousands of buzzing individuals. I couldn't see what insect it was - it was literally a huge buzzing cloud of dots. Bees or what? I've never seen anything like it in my life in all my years of rural living. I didn't think bees made such huge swarm clouds?