In article
,
RichA wrote:

What else could it be?

Anynumber of 1,000 things inside a very complex device. To assume it's
the frame material is ridiculous.

Still paranoid, eh?

We don't really know, do we?

you certainly don't.

If it's residual debris from unclean
assembly, then Nikon looks bad. If it's an ongoing shedding of dust
and pieces due to wear of ill-fitting mechanism parts, Nikon looks
bad. If it's some kind of lubricant that being flung around because
of over-lubricating or poor design, Nikon looks bad.
So tell us what you think it is?

it's not because they chose plastic.