Cat makes whistling sound when she breathes

So I “adopted” a stray awhile back and ever since we’ve had her she makes this weird whistling sound through her nose when she breathes sometimes. I never really questioned it but now I’m starting to worry it might be something serious. She doesn’t seem to have trouble breathing and it isn’t a gasping or wheezing sound. She seems perfectly fine otherwise which is why I never really bothered to question it until now.