I don't know if a dog's teeth chatter when it is afraid. Most likely you would see that in his 'body language' (like tail down and tucked in, or finding places to hide, or 'clinginess', following you around every move you make, etc)
Dogs do often 'chatter' their teeth when they have picked up an extremely interesting scent, such as a female in estrus. Even neutered males will do this, and age is no boundary to this instinctive behaviour. Does he start doing this after smelling things while out on a walk, then keep doing it for a while afterwards?
I doubt it would cause chattering, but has the vet looked at his teeth recently?
Hard to tell why he is doing that. Seems like he is okay, otherwise. I have often seen teeth chattering in many dogs over the years. I have a Greyhound and many of them do it often, usually when they are excited, etc. Nothing to worry about, they just do it.
So, no definite answer from me but you know your dog so I would keep an eye out for anything that may prove to be different with him.