When the viscous coupling on my fan was loose, it banged and emitted a tinny sound a little when the engine started up and when I turned it off.

Check the fanblade for wobble, with the engine turned off, of course. That could be the problem.

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Semibodacious Transmogrifications a Specialty

1990 300D 2.5 Turbo sedan 171K (Rudolf)
1985 300D Turbo TD Wagon 219K (Remuda)

"Time flies like and arrow, yet fruit flies like a banana"
---Marx (Groucho)