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.
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"