So I completed the rebuild, replaced the tranny/diff on the engine,
reassembled all except the axles.

Now I've filled it with fluid (gowesty's atf cooler--still empty from
the flush--has yet to open, probably) and run 'til it's warm and put
it through the gears to fill the valve body ways. No leaks, the drive
line behaves as expected for a successful rebuild.  I'm about five
quarts in, and the (warm) fluid comes up to minimum on the dipstick.

But when I put it into park I get a rusty gate sound for about a
second.  This doesn't grow less loud or long with repetition. Looking
at the internals of the tranny, I'd say it's good as new, no wear at
all.  pictures made during disassembly are here for reference:

https://drive.google.com/open?id=0B62_BQnCIJQ6VmtLTVZ5NmthNms

(I'm sure I marked that directory shareable, let me know if it is otherwise.

What is it?  I'm scared, since I've never even lookekd inside a AT
before, or any transmission.  Any clues from the experts in this mob
of experts?

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There's no .sig like the present