Dont know much about 87 model but on mine 85 300D Turbo, the high pitch sound is associated with fresh air recirculating valve. You will find some past posting under my name MVK regarding this. Many folks disable it. I do that once in a while when I go for a long trip to assure that my cruise work fine. As they somehow share the Vac.

This applies to my car yours might have different reason. Don a search under whisle sound turbo area and that may help you.

WELCOME TO THE FAMILY..
MVK

MVK

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1985 300D Turbo 179,000miles