I have encountered something very similar on both Audi A6s my father once owned (both V6s). As you say, metallic, rattling chain noise from outside of the car, particularly apparent when driving next to a reflective surface (eg a bridge or wall) with the window down.

Does it come on only when the engine is warm?

I think on our Audis (Audii?) the problem was a catalytic convertor heat shield. These thin and lightweight pieces of metal are particularly prone to resonating at a high frequency. The movement need only be tiny and may not be apparent on inspection and shaking by hand.

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JJ Rodger
2013 G350 Bluetec
1999 SL 500
1993 E300 diesel T
1990 190