The current generation of Powertap road power meters (the wireless 2.4GHz ones) mostly come with an aluminium alloy freehub body.  After not very long this happens to them.  Sucks ... It's because making an alloy freehub that works with both 9 and 10 speed Shimano cassettes compromises the design of the freehub.  Of course, the weight weenies want light hubs ... so for the sake of 80 grams (I weighed both the alloy and the steel freehubs today) all but the bottom end Powertap comes with this stupid alloy freehub.

But!  For around $200 or so (in Australia, from a Trek dealer) you can get the PT Elite+ freehub, which is steel (and ... yes ... 80 grams heavier) and swap it into your higher end hub and eliminate the problem!  Win!  It should not cost what it does, but that's not something your LBS can do anything about, wholesale these things are insanely expensive, but they are available and they mean you can swap cassettes with just the one chain whip!  Nice .. when something works like it should.

Enough ranting .. My PT 2.4 is now upgraded to a steel freehub and I'm happy about it.