It is a lack of grease on the splined output shaft and the corresponding female spline of the wheel AND the taper at the inner end of the splined shaft. I removed my rear wheels on Sunday and undertook the necessary re-greasing. Splines were bone dry.
In my (limited) experience, driving in very wet or hot weather definitely exacerbate the issue. I can also confirm that the louder it gets the less grease is in place and by implication, a more rapid wear pattern is likely to develop. I hasten to add that this is an assumption (but not unreasonable) on my part.
I'm afraid it's a case of periodically removing the wheels (when the clonking becomes audible again) and reapplying graphite grease to the necessary areas. I would have reasonably expected this to not be necessary between services (based on the MMC recommended service intervals), however, this has not proved to be the case.
