The bicycle is a Jamis Aurora. It uses a mountain bike rear cassette 12-32. The crankset is a Shimano Ultegra triple.
I have this weird knocking noise on the rear cassette only when it is on the third smallest chainring. Any other chainring and any other front chainring selection is OK as long as I don't crosschain of course. The chain is not hitting the FD. The knocking happens on both the largest and the middle chainring in the front.
There is no visible freeplay on the hub or the cassette. The RD shifts perfectly with no RD noise.
Thanks for all advice.