Odd that it only happens when backing. You don't have any other symptoms?
Check the obvious first: Make sure all of the wheels are square on the hubs and that all of the lug nuts are tight.
The hubs are held into the suspension uprights by bolts on the inboard side, and these have been known to loosen. Check to make sure they are tight.
The steering link could be damaged or loose. This will be more noticeable in reverse due to the caster in the suspension geometry.
A quick check for all of this is to jack up one front wheel at a time:
Grab the tire at the top and try to move it toward and away from the car. There should be no movement.
Grab the tire at the front and do the same thing. There should, again, be no movement.
Then spin the tire to see if it is turning true. There should be no variation.
Sky NA 2007 Midnight Blue
Sky RL 2008 Carbon Flash - Naked