He's 13 and does this quite a lot.
At the beginning of the year the vet said he had awful teeth and head-shaking could be a sign of pain.
He had most of his teeth taken out and it stopped. The few that he has left are clean and his breath doesn't smell.
But it's started again.
He eats normally and is happy and lively.
He doesn't go out much - just in the secure back garden and for walks off lead with me. We live in a busy area and he's a rescue, previously exclusively indoor cat, so I can't trust him to wander. He walks alongside and has a mooch.
All that means is that I know he doesn't come into direct contact with other cats. I also know he doesn't catch birds or mice - more's the pity because we are currently sharing the house with a mouse that he's very Zen about 
I've checked for ear mites and he has flea medication - not that he seems to have fleas either.
Any ideas before I take him to the vet?