My '85 300D had the same noise/whistle. After numerous questions and posts, I dropped a BB (by the way, BBs are 4MM. The same size the service manual says to use to block vacuum lines for testing purposes) into the vacuum line to the ARV and the EGR, as well. Presto! No more annoying squeal/whistle! I recommend this to all.
While I'm posting, I'd like to thank everyone on this forum for all of their help and knowledge. I have learned so much about my car just by reading so many posts on this site. Merry Christmas to all of you!

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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)