I had that with my 87 300TD. I wound up replacing the belt itself. Believe it or not, the squeal went, even though the new belt was the same as the old.

I also replaced the tensioner, but the problem with mine was that the shock on the belt tensioner had worn out, and the tensioner was rocking slightly. I wore out two belts inside a year before I realized it was the tensioner.

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1987 300TD wagon 193,000 miles