The center vent pod is most likely evacuated through a restrictor in the Y connector(look for a red dot or band on it) which I think was used by MB to prevent noise from the vent door rapidly opening and closing, annoying the well heeled owner. With the restrictor in place, a very small leak in the diaphragm or pod will prevent it from opening as the restricted vacuum supply cannot overcome it. Sometimes it may open part way but not fully. Movement of the piston/diaphragm might also open up the leak, explaining the clicking that you are hearing.
Peter