WordNet sense 1 (the state of being or appearing to be actively engaged in an activity):

busyness

WordNet sense 2 (an Islamic fundamentalist group in Pakistan that fought the Soviet Union in Afghanistan in the 1980s; now operates as a terrorist organization primarily in Kashmir and seeks Kashmir's accession by Pakistan):

Harkat-ul-Mujahidin, HUA, Al Faran, Harkat ul-Ansar, Harkat ul-Mujahedeen, Movement of Holy Warriors

WordNet sense 3 (a humming noise):

humming

WordNet sense 4 (be noisy with activity):

buzz, seethe

WordNet sense 5 (sound with a monotonous hum):

thrum