My Hope used to make this noise, she would wake the whole house up doing it in the night! I've always taken it as a sign of distress. When I went to investigate why she was making the noise it was usually because one of the others had tried to get in her cosy with her, or pushed her away from the food bowl. Strangely, the others never seemed bothered when she was doing it, they just carried on with whatever they were doing and ignored her!