My 10 year old CKCS has been chattering his jaw for a while now  he has a grade 3 heart murmer but is otherwise in good health.  He occasionally jumps up suddenly runs around and turns over to rub his back but doesn't seem to be in any pain, probably early signs of SM ?

Have you had his teeth checked, my Mams dog was chattering a lot, the vet suspected a brain tumour but turned out he had a bad tooth at the back and it fell out itself, the chattering stopped as soon as the tooth came out. He never drooled so didn't suspect a bad tooth and they looked okay when he was first checked.

The simplest things are often the most likely.
Check anal glands and have his teeth done.
Syringomyelia is characterised firstly by PAIN ( often phantom type pain) your dogs 'signs' seem to be more discomfort in the mouth.