I have a 2 year imd I hatched myself that does a whistle everytime she exhales hard. She's had it ever since she came out of her egg. For some it's a natural thing for others it could be a peice of stuck shed. And then you can have whistles and pops as beginning RIs. All you do is bump up the temps a few degrees and watch them to see if it's progressing. But when it's a developing RI its not every 15 min it's every breath in and out. You were smart to go to the vet. Glad to know your kiddos alright.
Morgan