Clunk noise on the 1/2 shift in an A999 trans.

Cley, when you drop the pan loosen the valve body bolts (10 of them) drop the VB & check if the accumulator spring has been removed or the accumulator piston has been blocked with a blocker rod. One purpose of the accumulator is to cushion the 1-2 shift.
Here's a picture of a 727 but it is very similar to the 904/998.