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

This is what I was thinking as well. In D 1st gear the rear band is not applied. The low/reverse drum is only held by the sprag. On the 1-2 shift the front band is applied & it is this action that causes the sprag to release & the low/reverse drum is free to turn in a clock wise direction. That is the reason why I asked if the accumulator piston had a blocker rod under it. One of the jobs the accumulator does is cushion the 1-2 shift, as the accumulator piston has to stop moving before full application of the front band is achieved.