Don't believe the shop. If the noise continues when the transmission is in Park, it can't be the differential. Someone doesn't know what they are doing or is trying to rip you off for unnecessary repairs. The early 300SD 116 transmission were known for stripping the ears on the front transmission pump. If that is the source of your noise, it would account for why the noise can be heard while in Park.

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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