I had the same squeak/shrieking noise on my '85 300D. Do a search on all my posts. I had the shrieking noise right after the trans shifted and load was off the engine. After posting and reading everything I could, I assumed this must be coming from the ARV (air recirculation valve on the turbo), or the EGR valve. By "accident" a 2.5MM BB (bought at the sports store) fell into the vacuum line to the EGR and the ARV, and wouldn't you know, the noise was gone. I 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)