Suggestion: Lay your BP on your bed or other flat surface, so that he is calm & you aren't holding him at all. When he is quiet (not moving), lean over & listen
near his head to see if you still hear the faint whistle in his breathing. Try to listen for a minute or two, at least. This helps to find out if he has any congestion
when there is no pressure on his internal organs (from being handled).

As bcr229 mentioned, this sort of noise can be much louder in a large snake; when my late BCI was about 7' or so, I was handling her for a couple guys who were
curious but fearful, & they heard it & thought she was hissing at them. She wasn't. It's just that she was wrapped over my shoulder, which meant that internally
her lung was folded over too, & this made for a "forced exhale".