I've found that my runner girls have quite a vocabulary. It's not all "quack quack."
They have different sounds they make when they're looking for me, when they are frustrated, when they see something they want, when they are content, when they are just staying in touch with each other as they forage in the garden, when they are frightened, and so on.
So I'd guess your guy is expressing some dissatisfaction, based on the circumstances.