Just got back from a road trip to NM. I noticed a sound on the trip. Under acceleration or decceleration, no sound. But, hold the accelerator in a partially open position and unload the drivetrain and a thumping sound occurred. It sounded like a tire with a lump but it obviously wasn't since it only happened under these precise conditions. I'm thinking it must be the center bearing on the driveshaft. Under load one way or the other it stays in position, but when free floating it thumps. Can anyone confirm or deny this diagnosis? Thanks.

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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