Noisy central locking
Unfortunately, the noise is inherent in the system: the lock motors are noisy themselves (not the motor itself but rather a disengaging mechanism inside), the rods and latches attached add to this, and the whole is vastly increased by the hollow door construction.
However, you can try the following: first, check the operation of the locks in all doors, check that the rods do not unnecessarily touch or scrape against any part of the door, grease them liberally. Then you could use foam rubber plates: either put a layer between the motor and the door metal or better yet, wrap the whole motor with this material. You might even try to insulate the inner side of the trim panel where the door openings are, but don't overdo as water inside the door should still find its way out at the bottom.
But don't expect too much. I did all these and the door locks are still quite noisy. The only real solution would be to find an electrically and mechanically compatible replacement motor with a radically different disengaging mechanism. As far as I know, unfortunately, other Citroëns have similar motors, too.