As far as the ratchets is concerned, it had done around 3,000 miles of hard riding and so was pretty well warn and had a good innings - much as htough I'd like to think it was just down to our power!
Thanks for the thoughts though re the noise. We haven't tried taking the bearings apart yet and lubing them so could do this to see if it helps.
We had thought that this could be being caused by the axel bending slightly under heavy load but dismissed this as a potential cause because once the noise starts during a ride, it never goes away. So while it does seem to be worse while climbing, it does continue when we get back on the flat alhough at a high frequency as you'd expect with the wheel spinning faster and increased cadence.
The thing that I perhaps find more confusing is why others haven't experienced the same issue? This has happened now on 3 different wheels that use the DT Swiss TDM hubs and on 2 different tandems. We haven't had any similar issue with any of the other tandem wheels I use (yet) which include Hope and Shimano HF08. We do around 7,000 miles a year which while a reasonable mileage, isn't massive and am sure others will do similar mileages and even more. The only other thing I can think of is that the terraine around here is very varied, with areas of flat, long steady climbs, long steap climbs, and shorter very steap climbs, all of which we ride most of the time. Although we can't ride the very steap and of the beaten track climbs in winter or when it's wet because we don't have enough traction and the rear wheel just spins. But maybe there is just something about the riding we do, combined with the mileage and way we are pedalling that is causing this issue to occur.
Anyway, thanks again and if anyone has any other ideas on how we could prevent the issue, short of replacing the hubs which have been otherwise reliable, let us know.
Cheers
Ian