Does she seem anxious about anything when she does this? Does she play well with the other two? Does she seem to see well when you drag a little toy for her to chase? Some people whistle or hum and don't even realize they are doing it. Maybe she has a little Siamese in her, they are very vocal. Sorry I can't be of better help but if she's a sweet, happy little thing I wouldn't be too worried. Who can really figure out what goes through kittens and cats brains anyway Welcome to PetTalk and feel free to ask anything at anytime. We may not know the answers but we will always listen