yea, my duck and my drake both purr, if I hold them and scratch their necks... kind of like a dog when you hit the right spot lol.
And my female is very loud, its a good thing my neighbors adore my ducks lol since I live in the suburbs. She tends to only be loud at diner time, breakfast time, pool changing time, bathing time, mating time, when she wants to play with the dogs, and of course egg laying time. She use to hide her nest by has picked it to now be right under my bedroom window. She I hear her before she lays every morning.
My drake is alot more muffled and is not very vocal, it was just the strangest noise I heard, when he did the donkey thing, but I have now paid more attention and agree it is before he wants to mate, or when they jump in the pool the 1st time in the morning. (aka when he wants to mate lol)
My drake does sound like he has a bark, but then again he was raised with my dogs, and doesnt know any better... he thinks he is a dog. (he even wrestles with them, which they just stand there and he wrestles lol, and will take their bones to hid them, and he will even run after them to try and play fetch) Poor thing even expects to come in with them at night, and when we go for a car rid, he sits on my lap, and wants to stick his head out the window. haha He's got the best personality even!!!