it's a random event that happens sometime in the beginning of any new game — in story, it's a military black hawk helicopter flying over the Knox County area looking for survivors.

The effect in-game is that the zombies in the world all gather around to follow the noise source, which controls and drives a gigantic crowd of zombies around where you're at. It can be very overwhelming, especially when you're just starting out and don't have much by way of structures built.