One of my 18 year olds does exactly the same thing, not always, but quite a bit, mainly at night and it sounds like she does the same meow/growl noise too. When I spoke to the vet about it he said she was probably going senile. She also kinda 'stares' into space. I know cats do that sometimes but there was something different about it - almost a vacant look. You know how the elderly can sometimes drift off onto another planet and start talking rubbish - its almost like she is doing that.
There was some kind of drug to help but it didn't actually stop her being senile, just helps with the 'actions'. The weird meow and the rocking were not harming her so I decided against it as I couldnt see the point of pumping her full of drugs if she wasnt in any pain.
Ask the vet, they will be able to point you in the right direction.