Hi again all
I decided that as my Elite was out of warranty I had very little to lose by opening it up and seeing if I could find out what was going on. An initial inspection with the top cover off showed nothing broken, and no obvious signs of anything wrong. The construction of the main wheel output shaft is such that there is a thick sheet metal plate that holds the thrust bearing, which is also bent in such a way that the rear two lower base mount bolts are threaded direct into the plate.
It's reasonably sturdy, but close inspection showed that there was a bit of movement when you applied some lateral force to the wheel output shaft. It appears that the whole assembly was able to move slightly when the force was applied, and the cracking sound occurs when the plastic covers around the rear or the mechanism moved.
By going around and judiciously tightening every screw I could find I was able to eliminate the movement, and when I put the whole lot back together and bolted it firmly down on to the rig mounts I am happy to report that the nasty cracking sound has been all but eliminated.
While I still fancy getting a DD1, I now can do so at my leisure rather than being forced to at short notice. With any luck there will be a sale that reduces the price at some point, and I can wait for that
Cheers
Les