Poke around here to learn about the hydraulic valve lifters. Maybe that is what you are hearing. Mine sound like clicking/tapping which is most noticeable at idle, but you keep hearing it somewhat as the rpm increases.

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1968 220D, w115, /8, OM615, Automatic transmission.
My 1987 300TD wagon was sold and my 2003 W210 E320 wagon was totaled (sheds tear).