Last time I had whopping thunking noises coming from the rear, it was exhaust hangers - some were broken. New hangers and she purred like new.

Check that first.

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Codifex
1981 240D ChinaBlue (Got her running with a donor engine.)
1983 300DTurbo w/sunroof.
1984 300TD manual sunroof. (Electrical Gremlins)