For a second, he thinks it's only the wind flicking through the rocks along the shore. That happens, he's found. The wind can sound like a whole lot of things, depending on where it's going and what it's stuck between. So he doesn't think much of the whistling, at first.

But when it doesn't stop, and he realizes the air is perfectly still... Joker stops as well, suddenly all too aware of his precarious footing on the rocks. There's definitely something in the water, when he looks, pale as the moonlight that's shining down.... but it's not any singing yet.

Well, the only thing he can do is whistle right back, mimicking the notes of his own song that are being whistled at him.