Are you talking the drip tube that comes down into the main compartment? It's most likely the defrost at work. Frost builds up on the coils. Defrost heater turns on and melts the frost, water drips out of the freezer thru the main compartment and into a drip pan underneath the fridge where it evaporates.

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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