I had a foster dog from the rescue who did that every so often. The first time I heard it, I checked his mouth to make sure there was nothing wrong because it was such a strange sound. When I took him to get his shots at the vet (so he could be put up for adoption), I asked the vet if it was a problem or what could be done. Vet said it was fairly normal and that a lot of dogs do it at some point-most people don't notice because they're asleep too(an excellent point!). It only turns into a big issue if it happens all of the time...like when they are napping, sleeping, even sometimes when they're awake. Seriously, doesn't sound like a big deal. Probably, just a big rawhide. :)
My rescue pomeranian does this at times. Usually after a long and busy day. I took her to the vet and many of her teeth have begun to rot, not due to teeth grinding but due to who knows what she lived in before.
Our vet told me to wake her if she is doing this to prevent anymore damage. Between grinding teeth and snoring she is a very loud sleeper.
I have a yorkie who has seizure disorder. Over the last 2mos. he has been grinding his teeth in his sleep but also is jerky along with them. He puts you in the mind of someone who has tourette symdrome. He has nervonus tics with head jerking backward and eye movement. I think it might be like focal seizures. It is not like the big ones he has. He is on 2 meds to controll them.He has been in diapers since april due to loss of bladder and bowel controll. He is like taking care of a baby. I got him "onies" to help hold his diapers on. I have had him 3 yrs. He was rescued. He was abused.He only weighed a pound but now he is 7 1/2 pounds. The vet says he was around 4 yrs when we got him. He is now 7.