There's a very haunting sound the Ringwraiths make at the very end of the Buckleberry Ferry scene: Frodo says, "How far to the nearest crossing?" Merry replies, "Twenty miles." And then you see the Wraiths riding away, screaming. Then the Hobbits are at the gates of Bree. It's almost a howl, really.
If you could get that Wraith howl, I'd appreciate it. Thanks!