G'Day Adam,
Noises are very hard to diagnose when you can't hear them
Possible sources:
Fan belt
Power steering pump
Power steering rack
Suspension bushes
The middle of the rack is a solid tube hence no moving parts to grate on gravel.
The crossmember does have a couple of spots (one at each end) which seem to be designed to collect dirt/stones.
Try to narrow down where the noise is coming from, have someone reverse the car with the bonnet open and you listening.
Try flushing the old P/S fluid.
Also lubricate what you can get to of the steering column.