To be honest, this is the type of thing that I would ask the vet when you go in for the yearly check up. Not somethinjg I would make a special trip for. That is assuming he is healthy in all other aspects. I am sure that there are some medical conditions that could cause this, but if he stays at a condstant weight, not gaining, and is eating, drinking, peeing and pooping normally with no vomitin or pain to be noticed I would not woory. I would just ask as a routine question at a time when you are going in to the vet anyway.
Just one person's opinion