What is this sound my cat is making?

by Anonymous

I adopted a stray cat, she's been living with me a few weeks now and when she eats, cleans herself or holds a toy with her teeth she makes a strange noise. Our veterinarian couldn't find an issue. We checked her jaw, teeth, lungs. Sometimes she also whistles a little when doing the same things. She has no vomit, cough or sneezing. Looking for your advice