That noise sounds a lot like the one I had. I assumed (after a lot of searching on this site) that it was the ARV. I put a BB in the vacuum line going to it, and it quit. Hope this helps.

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'85 300D 190K miles
running on B100 Biodiesel since 156K, SCRATCH THAT! I can only find Biodiesel at 1 location in Houston! (not very convenient)