Thank you for the advice, in response to your suggestions.....

-Check rear q/r skewer tension .....it is tight

-check the machine screws are properly tight on the torque reaction arm.... the bike was built for Rohloff, the torque arm seats in a slot in the dropout plate (no screws)

-Check the slotted/circlip-retained drive sprocket... wheel off bike, the sprocket has no noticeable angular or side-to-side play

It occurs to me that the problem cannot be the seating of pawls to the freewheel axis (unclear proper terminology for Rohloff). The crack sound occurs only once after a shift and with initial pedaling. It does not occur after coasting and then pedaling without changing gears.

Lately from our Rohloff, I hear a crack noise after shifting on most if not all gears. I suspect this is the main drive pawls properly seating when significant force is applied to the pedals. I not sure if pawls is the correct term for the power engagement point on the axle of a Rohloff (if possible, please point to the correct object on the Rohloff exploded diagram). This suggests that the engagement shaft (where the pawls dig in) has developed some abnormality.

I have changed the oil yearly, sometimes twice, and oil leakage is not a problem. The hub has 40,000 km and is on a tandem. Is this a problem that others have experienced?

Thanks, Mike

Hi Rotavator. I have sent you a pm.

very good 

I shudder when I think of the amount of work, the time consumed. Your reward will be in another world, Dan. Meanwhile, I add my appreciation.