Did you find a solution? My brand new Ultegra rear has the same problem. Two clicks each rotation of crank when pedalling hard, especially out of the saddle. Happens especially when moving very slowly up hills. I thought it was a crank issue but it seems to be coming from the rear wheel. Doesn't happen when coasting or when pedalling on the flat. Could it be the cassette or the hub? Or flexing making the spokes click? Any suggestions from battle hardened click doctors appreciated.