2000 Camery noise when backing

I only hear popping/knocking when I back up, nothing when I turn only backwards, any suggestions? boots? cv joint?

2000 Camery 55k miles, no trans issues, perfect changing gears

Thanks

Actualy this is a sign of bad CV joints, but with 55,000 miles... Are you sure that nothing is on the way? Mud guards... May be something is stuck under the car.

Sam