Generally when a wheel fails you get a grinding noise that repeats with every revolution of the wheel. We had no noises like that at all this trip.. instead we had something that sounded more like little chiming noises as the wheels rotate, but continuous rather than once per revolution.
I jacked up the dome and removed the wheel to find out the cause of the problem. In the past we have had failed bearings which have caused the repeating noises.. but maybe this time the failure wasn't complete and we were listening to the bearings bouncing around under the wheel dust plate.
To my surprise this wasn't the case. What had actually happened was that the metal crimps between the two sides of the hub had failed. Each side was complete with it's own bearing in perfect working order.. but the two sides were no longer connected. The noises from the wheels were the crimped rings of metal which should be holding both sides of the hub together.
We have spare wheels on site, but they are all of the same construction as the failed ones. To remedy this I have replaced the wheels with new ones to be safe, and also bolted (with nylock nuts) both sides of the hub together. We'll need to make sure they are doing OK on our next maintenance visit and maybe find a new supplier in the UK with stronger wheels.