Late to the thread but I am having this same issue lately. It has been fine for about a year but in the past week I get this clicking sound, just like you said its mostly in the 42t ring, sometimes the next one down. It only happens when I putting down power, generally going uphill. I can't replicate it really while on flat ground or in my street (maybe cant put enough power down in that gear on flat ground?)
Anyone have any luck with figuring this one out? I stopped by a shop and they did not really have any guesses. It shifts great in the stand, hanger is aligned well, and cable/housing is in good shape. They suggested I could change the housing and shift cable to see if that helps, but I don't really know where to start.