I know you said that you had your wheels serviced, but just thought that I'd throw this out there. I had a pretty loud clicking on harder efforts (which for me is over 100 watts mostly when standing heading up a hill. turned out to be the cassette was just not quite tight enough. Even though I had greased and torqued it correctly there was just enough movement to make a loud click. Took it back off, re-greased and re-torqued and all was well with the world.