Foster kitty making whistling/wheezing sounds when sleeping - what could it be?

Disclaimer: we’re going to make a vet appointment tomorrow, just wondering if anyone had any idea as to what it might be.

Hi, me again

Our foster kitty makes a whistling sound when breathing out while sleeping.

The first time I heard it she was awake though. It was the very first night she had with us and I startled her when she was exploring the room. Her breathing was very whistly. I read that it was normal for some cats to have in stressful situations so I wrote it off as her new environment being the cause.

But the last few days I’ve been hearing it when she’s sleeping. Only on the exhalations. Her breathing doesn’t seem obstructed or as if she has trouble getting enough oxygen at all. It just sound like when people have a tiny booger stick if that makes sense lol

There’s no discharge from her nose or eyes, no coughing or sneezing

The shelter checked her for worms and all the regular stuff. Could it be asthma?