Another possibility.

Sounds to me like a bad or stuck solenoid. If it happens again, try tapping on the starter with a hammer. Might not be disengaging after a startup.

If this turns out to be true, the mechanism is probably able to be lubricated. Or else replaced.

I've never had to replace my starter on any Benz I've ever owned. But, my current 300D's starter is starting to make some moaning noises so... I might get some firsthand knowledge soon.

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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)