Would you send me the document about the shifting problem. Thanks

I have a 09 Sierra with transmission issues as well, for the first 4 years not a sound out of the transmission. As it got into the 5th year it started a nasty clunk when shifting from 3rd to 2nd and 2nd to 1st, always at 40k & 20k. The GM dealership at first claimed it was a normal thing, I would not buy any truck new that has transmission noise, they then took it apart and replaced many parts and added a part that the factory frogot? It still makes the same noise, and to top it off the noise is worse in the summer when its hot, you would think with the metal being hot the clearances would be tighter? In the winter it's not as bad, I am running the factory synthetic oil, this is the only sound the 6 speed makes.