Hobbs used to do that when he would get really excited. It sounded like he was trying to cough up a hairball or something. I was really worried at first that he might have a collapsed trachea, but his cough went away. I would not think that her harness would cause a collapsed trachea. But, if you are really worried about it then take her to the vet. Sometimes dogs just need to cough like humans do. Maybe she is just coughing
Good luck and I hope she feels better soon!