Can you see what she's doing when she makes that noise? Occasionally I'll hear that from one in my rack, and when I investigate it's because one of them has its nose crammed into an upper corner and is trying to escape... often after going potty in the tub. Or if I've thawed rats on feeding day I'll hear it, the ones that are really hungry will have their noses crammed up against the ventilation holes in their tubs.