It was a shot in the dark sometimes the injector pump can make a noise and as it is tied to the rpm of the engine it will be in sync with acceleration and match your engine. If I remember correctly the vacuum pump is also right behind the injector pump, so either could cause the noise as they both are connected directly to the engine and will vary with engine rpm. The hard part is isolating the sound, I think you are on the right track there to fix this you are going to have to pinpoint the sound exactly, not easy on an MB diesel.

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Martin Ingram
Colorado Springs
2005 320 CDI
2006.5 VW Jetta TDI
1991 560SEL (179000 Sold)
1972 280SEL 4.5 ('The Lead Sled' 320000 miles when sold.)
1972 220D (225000 when sold)