In manual transmissions, the reverse gears use a different type of gear teeth than forward gears. Forward gears are helical gears, which have teeth that are pointed at an angle. When the gears rotate, most of the load is evenly spread due to the angles.
Reverse gears are cut into spurs, which don't absorb the load quite so well. As a result, there is more of a whining noise. The reason reverse gears are spurs is because reverse requires an idler gear to prevent accidentally shifting into reverse while moving. This is what is known as the gears not being synchronized, and this is also why you cannot shift into reverse when you're moving.
What you may find interesting is that my 1987 Ford F-150 4x4 makes this same sound when moving forward in low 1st gear. The 4 speed granny low first gear is also using this same technology as it's only designed for towing or going up or down really steep hills.
For more information on transmission gears, check out Why does my manual transmission car make a loud whirring noise in reverse?