BINGO. That's the banjo bolt that reads the intake manifold pressure and controls the ALDA. I think it's possible you're hearing the intake manifold pressure leak out of the hole where that banjo bolt should be installed on the back of the intake manifold.
It's not a big worry but it does mean that for all intents and purposes the turbo is not providing much boost.

That explanation doesn't account for the rhythmic sound though.

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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