It has become very normal for Sierra to whine all night long, which, as I'm sure you can imagine, has been rather a drag for the month and a half that it's been going on. Last night, after about three weeks of the "just ignore it" strategy, we finally cracked and went in to make sure that there was nothing bothering her - we changed her bedding, put the sheet down to make her crate more den-like, shut off all the fans and computers and closed the windows to make sure that she wasn't being bothered by sound outside. She was delighted to be let out, even though we wouldn't let her go and do the things she wanted to (look out of the sliding door in the living room is my guess), but of course scant minutes after we put her back to bed she was whining again.
As we'd already responded to her, I tried one last thing - I made the "uh-uh" noise that we use to tell her she's doing something wrong, very loudly so that she could hear it. She shut up right away. I'm kind of kicking myself that I didn't try it a month ago, but all the advice has been not to respond to the whining - we're normally very careful to only go and check once she's quiet for a while.
We'll see how she is tonight, and if she whines again, whether "uh-uh" can quiet her down again.