Last spring my 3-yr-old male bp Yonni developed an ominous whistling when he breathed. I rushed him to the vet, who examined him and did a lung culture that turned up negative. The vet said either wait and see or give him a course of antibiotics, which involved numerous injections over a number of days.

Since the culture was negative we decided to wait. And he seems fine -- eating, sleeping, pooping, curious, happy to come out of his cage and hang out. He does not lie around with his head up or seem to have any difficulty breathing. But the whistling has continued. Sometimes it's not noticeable but for the past few days it's been really really loud.

I increased his humidity two months ago to about 50 percent, and he has a good thermocline. He's on aspen with moss tucked into the corners. Any suggestions on why he's whistling or steps we might take to make it stop?