Bailey whistles sometimes when she sleeps, so it's pretty normal.
I wouldn't worry too much unless your kitty is also showing other symptoms that she's sick, i.e. shivers, lethargy, not eating, etc. If your kitty has any of the above symptoms, then I'd head to the vet ASAP.
For future reference, the above symptoms can be a sign of pneumonia which can be fatal for cats if not treated. I caught it very early when Bailey had it so a course of antibiotics and vitamins was all she needed.
WR