Could very well be a combination of nose rubbing and a bad shed.
If you notice any other RI symptoms, specifically mucous coming from the nose or mouse, bubbles, or open mouth breathing, then make a vet appointment.
As a personal rule for myself (I'm not a vet!) I don't start thinking RI until they show another symptom other than whistling or other strange nose noises. It just isn't enough for me to warrant going to the vet. I have a pewter that whistles when he breathes heavily. he has been doing it ever since I got him and it has not changed in between sheds. He just has a whistley nose! At first I thought something might be wrong, but after a couple months I figured that is just how he is.