Strange sound from Yellow-neck

Whilst checking the dormouse nest boxes at Coed Pendugwm Nature Reserve the other day, I found two Yellow-necked Mice in one box, one male and one female. Once in the bag, the female constantly made a sound which I have never heard any mouse make before.

I'm guessing it was a response to the stress, maybe even a warning to stay away. I would be interested to know whether anyone else has heard something similar.