dog has a hacking cough

I've got a dog almost the same way, he started coughing when I got him...... 6 months ago. I took him to the vet twice as I thought it was heart worms too but he was heart worm negative, doc said it could be similiar to a kennel cough and gave him a round of anti biotics, it helped for about a month and back again, second trip to the vet he gave him a shot (it was bad this time, more than when he just got excited) and that really helped but to this day he still will "hack" when he's excited on the chain. I've even switched from Diamond to Victor food with no luck. So if you figure out what it is for your dog I'd like to hear.