For the first time ever (to my knowledge anyway) Max just kind of howled in his sleep. It wasn't a full-on howl, but it was noise from his mouth and it wasn't barking or whimpering.
I wasn't sure what it was at first but then I saw his mouth moving. I said his name - woke him up - and he stopped.
I've now heard him bark, whimper, and howl in his sleep.