(New Delhi, INDIA)
Although not frequently; but occasionally I have caught my 16 month old rottie, Shadow, howling while sleeping.
I call out to him and he too looks surprised at what he is doing and immediately stops howling. Is there anything wrong in his behavior ?
I'd say not to worry, dogs seem to dream just the way humans do and it's not unusual for them to whimper, cry, bark, 'run', even howl in their sleep.
It's often more obvious in puppies, but adult dogs do it too.
If Shadow seems to be perfectly fine once you wake him up, and as long as he doesn't show any signs of discomfort or pain when awake, then I would personally not worry about this behavior.
However, if you are still concerned, or if you feel that there may be a health problem of any sort, then I'd advise discussing it with your veterinarian. It's always better to be safe than sorry.
Best of luck. Shadow is beautiful and I love his smile!