Sounds like it's the vacuum servo for the footwell vents. Remove the kick panel under the driver's side dash. To the right of the accelerator behind the heating ducts is the servo for the footwell vents. Put a mighty vac on it and see if it holds vacuum. When mine failed it was a leaking rubber connector between the plastic line and the servo itself.
Dont mistake the defrost servo for the footwell servo. The defrost servo is on the side of the ducts about a foot in front of the accelerator.
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13