As the others said, loose skin around the nostrils can create a whistling sound and sound like wheezing from an RI. If you'll soak him in tepid water (NOT warm to your touch, but not cold either) for half an hour or so, then let him crawl through a damp towel repeatedly, you may be able to get the skin off without bothering him too much. If he's still making the sound when the skin is gone, it's time for a trip to the vet (see www.arav.org if you don't already have one).