cold air through center vents with heat on 94 E320

[skybound]

Hi all,
I have a problem with cold air coming throught the center vents when the heat is on all other vents. When it is cold in the car I set the thermostat and the heat works just fine with the only problem being cold air coming through the center vents. I messaged one of the moderators of this board and was told it was most likely a vacuum actuator. Has anyone else had this problem with a 94 E320? I have been searching the net and it looks like if it is either what is called the monovalve or an vacuum actuator, in which case I may have to remove the dash.....UGGGGH, not something I look forward to. In addition to this I noticed recently that when I start the car I can hear one of the doors of the ventilation system moving and either closing or opening (I can't tell which, just a sudden thunk near the center of the dash). I know that most problems with most models are common and maybe someone else hear may have solved the same problem here. Any help is appreciated. If indeed this a vacuum actuator gone bad, are there any write ups/post with how to replace the actuator? If not I will have to post one myself if it comes to it.