Why is my dog having muscle spasms?

My new dog (recently adopted and don’t know background) has what appears to be constant hiccups… maybe some kind of tic or spasm. But it is very rhythmic like hiccups. It never stops even while she is sleeping. I brought her to a vet who said she had never seen it and wasn’t sure. I’m hoping someone has seen this before. Any information would be helpful. It seems to be more on the left side of her upper body.