Hi,
Has he shed with you yet? They can whistle when breathing heavy - most usually this is cased by either stuck shed in the nostril or the nostril getting constricted by the fluid layer that builds up between the new and old skin as they prepare to shed.
We only get really worried about things like RI's when there are other symptoms like mucus/ snot in the waterbowl or smeared on cage furniture or another sign of breathing problems like sitting with the head raised and the mouth open for extended periods of time.
del
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