A couple of people have reported the very same problem with the pedal noise especially on German forums, but with no serious cure.
Most say to spray WD 40 on the booster...but this is nonsense because there are no moving parts outside.

The squeaky noise is definitely NOT generated by the pedal inside the cabin. I have tested this for a long time. Pressing down the pedal is moving the piston of the master cylinder crossing the booster. The noise must be generated somewhere in the booster area and transmitted through the fire wall back to the cabin.
The very same noise is audible actually on the engine side of firewall if not louder.
I am ready to take the master cylinder and booster off to solve the problem.
Any suggestions?