its the signal flasher indicating that the lights are not working...
if one light isn't working(it senses the load/draw from the lights) it will just blink fast, but if both are out on one side the i just gives out that god-awful buzzing sound.
now if you have done and LED swap with out putting in resistors then you will get this -i put LEDs in the rear(was going to do fronts, but didn't have resistors at the time so i couldn't stand the buzzing) so mine just blink fast.
also if you only trip the indicator lever part way it will do the same thing, make sure you are depressing it all the way down and you should be fine