Ducks quacking at night is fairly normal and can be caused by a few typical factors. Ducks will quack if they sense the prowling presence of a predatory animal such as a fox.
Ducks are stereotypically known for their distinctive quacking sounds. But interestingly, you’ll find that not all ducks quack. In fact, most of them don’t quack.
Keep your ducks inside their pen through the night. The ducks might feel safer from predators inside their pen and not feel the need to alert each other with noisy quacks