On Wednesday I wrote about susurrus, a word from Latin that means "a hum, whisper," according to Merriam-Webster Online. M-W's entry on susurrus also suggested looking at its entry on swarm. Between that and the American Heritage Dictionary entry, I found that swarm is thought to be a Middle English word from Old English swearm, meaning a group of bees; it is akin to Old High German swaram and probably to Latin susurrus. The OED concurs and adds that "The root is usually identified with that of the Sanskrit svárati, meaning sounds, resounds, and svará, svára meaning sound, voice."
However, the OED also says that swarm's etymology might be entirely different, related to the movement of the swarm, not the humming sound it makes:
"But the etymological meaning may be that of agitated, confused, or deflected movement, in which case swarm and swerve might arise from parallel formations on the same base." The OED cites "the parallelism of swarm and swarve (both can mean 'To climb up a pole, tree, or the like, by clasping it with the arms and legs alternately'); the Norwegian dialect svarma, meaning to be giddy, stagger, dream, and svarva, meaning to turn, go in a circle, stagger, be agitated; .... also the meanings of the German schwärmen, meaning to swarm, rove, riot, fall into reverie, rave."
The OED's climbing meaning for swarm (and the synonymous obsolete word swarve) is "of unascertained origin. Perhaps originally a sailor's word borrowed from the Continent, but no trace of the meaning has been discovered for phonetically corresponding words."