Dog has coughing, doctor at a loss

12 year old Pomeranian. He has no heart issue, lung issue, everything looks and sounds fine except he’s diabetic bht it’s controlled (we think) and he’s a normal weight. No heart failure issues. Our vet says he has no clue. They notice slight tracheal issue, like fluid or something. He just coughs and wheezed a lot. But he will go outside run around be fine and then just coughs forever sometimes. It’s like he’s working himself up. Any thoughts we could throw the vets way? Last X-ray was a week ago. Nothing to speak of.