I can appreciate the acute dyspnea you're noticing - unlink panting, it appears the bird is having a hard time getting air into its system. This could be due to several things, and without a veterinarian, it is hard to determine exactly what might be the problem. With young animals, ingestion of foreign material can be common which can result in inhalation of the material into the airways which can obstruct the breathing airways.
Such as case may be identified by opening the duckling's mouth and checking for foreign matter in the nares, glottis, or choanal slit, and by palpating along the duckling's neck for abnormalities.  If you have access to a stethoscope, place the diaphragm on the bird's dorsal aspect, and auscultate for abnormal sounds (clicking, rattle breathing, etc).
Respiratory infections can be a problem in young birds, but if that were the case I would suspect a more routine and consistent display of the symptoms, as well as additional symptoms such as sneezing, or coughing. Since the wheezing is sporadic it may be an environmental problem at play, lack of deep water, etc.
Ensure the duckling has adequate deep water throughout the day, bedding/feed free of dust, and an environment free of pollutants (hair sprays, smoke, etc.