Muscovy ducks make a breathy hissing sound, but it's not at all aggressive. It's more of an excited "hello". Sounds like ha, ha, ha - low and breathy. The black Muscovies do it more often than the others. Don't know why since my research reveals the colors are random. But, the black ones definitely have a more pronounced ha, ha, ha sound, while the ducks with more white and green have more of a hissing sound.