It's nothing to do with the gearbox, the other way round.
It's the brake calipers themselves.
A Q system , like any automatic car will creep forward in drive so you have a small amount of torque that the brakes are having to hold back... It should be silent when you release the brake pedal but invariably like all automatics the brake calipers have a lot more to do in comparison to a manual car and the parts wear on them.
There is a cure to the creak/knock. New quality pads help a lot but the creak will return after the initial bedding in process.
The problem is the main caliper body twists and as a result you end up getting wear in the rubber boots that hold the stainless steel slider pins, they go slightly oval and that's where the excess play comes into effect.
You can buy a repair kit cheap enough and it is quite easy to swap the warn parts over as they are push fit
Here is a link on how to by one of our members: Alfa Romeo 156
It is the two hard rubber sealing bungs that are causing the issue
You can double check by removing the road wheel and grabbing hold of the caliper body and move it side to side, if they are worn you will see a twisting motion.
They will still operate ok for now but vibration will start to occur on heavy braking at high speed and then you will end up getting a metallic clunk that sounds like its the suspension dropping off!
This is caused by the caliper body moving so much that the top and bottom retaining lugs of the pads that are making contact with the stationary frame of the caliper over road imperfections and will drive you crazy....
At this point you will need new pads also as well as the repair kit