Collapsed trachea or reversed cough?

[FryFry]

When Leo gets excited on the leash or starts to pull, he'll start making this coughing/hacking/small honk sound. It only happens when he pulls on the leash, or when he runs around and plays. He wears a harness, and has never worn a leash on his collar. I noticed at the dog park when he's running around he'll make this sound too. He doesn't seem to be having difficulty breathing, though. He will run and play for hours on end with no signs of distress. Like I said, this only happens when he pulls on the leash or runs around. My fear is it could be a possible collapsed trachea?

Another issue is I think he might have a luxating patella. When I first got him, he was tested, and checked but results came back clear...well a few months ago, he had jumped off the arm of the couch and I took him into the vet. She did a physical exam, moving his back legs around and whatnot, and told me everything seemed fine. Nothing was popping out. She told me to give it a few days and if he started to limp again or act funny, to bring him back in and get xrays done. Well, needless to say he was fine and stopped limping. This probably happened when he was 6 months old. Now he's 10 months old. A few weeks ago, he was running out in the kitchen, when all of a sudden he stopped, lifted his back leg and wouldn't move for a few minutes. Then bam, he was running around again. I thought maybe he just turned the wrong way. Today at the dog park he was playing with a Min Pin/Yorkie mix when suddenly he stopped, let out a slight wimper and when he stood up he was holding his back leg up. I picked him up, and decided to leave. When I put him down to see if he could walk, he started to walk perfectly fine. Now I'm a little scared about BOTH possible things. Zoey has a vet appointment on monday for allergies, so I think I'll make Leo an appointment when I'm there.