What is the humming noise during the Football Confederates Cup in South Africa?

All the answers so far are completely off the mark !!! Simples..Its the humming bird mating season and they're all mad for it so the males swarm in their millions looking for a freebie and cause this horrible noise while sizing up the crumpet hummingbirds, it only stops when they are all satisfied ually and the mating season lasts another 2 monthe so get used to it..hhhmmmmmm