Don't know temperatures where you are but downshift clunks are often caused by vacuum leaks. You can check to see if those leaks are in the climate control system by pushing in the far right button of the climate control to turn it off. If the clunking goes away, the leak is in the climate control system somewhere.

__________________
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