I thought I heard a definite quack at the end too!These could still turn out to be to Drakes I'm not saying they can't be...
there is a little window of time when their voice changes from their baby peep to either there quack or the rest that is fairly hard to tell it can go either way. But when I listen to the video again I am sure that was a quack at the end. a drake will typically never make a quack sound they go from a peep to a whistle type sound to a frog like sound.