Thought I had posted conclusion to this problem, but seems I did not.

Noise had gone away, but then came back and not just while starting. This actually helped diagnose problem.

I squirted some water on the alternator belts and noise went away briefly. This was repeatable. So I sprayed on some belt dressing and noise has now been gone for a few days.

Had this problem before but had forgotten. The noise was not a belt squeal, but more of a metallic sound. Could be some misalignment of the alternator sheaves. Belts are quite new, so I don't think I will fix it just now, seeing it ain't broke Shes just purring along again!

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Graham
85 300D,72 350SL, 98 E320, Outback 2.5