85 300TD. Not loud enough to be a valve but increased with engine rpm. Seemed to be coming from left front of engine. Finally started putting my hand on the engine. It was the cruise control actuator. Not sure what was going on inside it. I don't use the cruise so it must have been accidentally turned on. Once I turned it off, sound went away. This post is just for information purposes in case it happens to someone else. It's close enough to the vacuum pump that it might be mistaken for a vacuum pump problem. I also thought briefly that it might be the SLS hydraulic pump.

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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