Lightsaber sounds
Apparently, the sound for the lightsabers was supposed to be silent at first, only later did they realize that some type of sound effect would be required. So they had to think long and hard about just what exactly a killer sword from the future sounded like. Then the idea hit them.
The sound department got hold of an old movie projector that made a certain humming noise and mixed that with a television set interface and a shieldless microphone. After these strange noises were recorded, they were put together to give us one of the most iconic sound effects of all time.