The effects of the immobulus from before still had a hold over the Whomping Willow. As the Ilvormornian became human once more (which, for the record, it was a pity there was no one around to admire the spectacle), the Willow, in stillness, grew agitated. It wasn’t like this was noticeable, since it was frozen, but the trunk gave a woody grooooaaaaan.

Don’t let yourself run out of time now, dear.

The branches began to shake. It isn't violently yet, but the intensity will only increase with time until the freezing charm wears off.