I know that all vehicles with a manual transmission make a whining sound while backing up in reverse. However, I've always wanted to know what that sound is exactly. Reason is, I own an automatic Honda Civic, and it makes the same noise in reverse. Saturns do this as well. The faster you go, the louder and more high pitched the noise gets. Anyone know what this noise is?

It is back lash due to play in the gears. Think about the gears you have seen. Some of them are straight cut gears, 90 degrees to the side of the gear. Some of them are helical cut, slanting to the side. The helical gears are less free to move back and forth in relation to each other. They are used where quietness is important. The straight cut gear are used where more noise can be tolerated to save a little money, like reverse gear.