Could you post a video of the duckling breathing?
The video below is a duckling panting due to heat:
You can appreciate the rapid breathes the duckling is making, in an attempt to expel warm air, and replace it with cool ambient air to lower its body temperature.
The minor breathing above is fairly common with large breeds of ducks such as Pekins and is not necessarily indicative of a problem - they are heavy breathers.
The video above is more indicative of a problem, as you can notice the chicks are rather trying to inhale air into their respiratory system, rather than get the air out in fast pants.