If this were a productive cough I would be quite concerned your girl could have heart disease, however we would be seeing other signs also like exercise intolerance and ongoing lethargy. In saying this, it would still be a good idea for your Vet to have a good listen to her chest to make sure everything sounds fine.
One of the most common causes of this symptom in little dogs is collapsing trachea, which is very common in milddle aged to older small breed dogs such as Chihuahua's. There are a few different treatments available including both surgical and medical and I would like you to have a look at this site for more information on the syndrome: http://www.peteducation.com/article.cfm?cls=2&cat=1611&articleid=410
It is also important to remember this could be due to an infectious cause such as kennel cough which does cause a dry goose honk 'hacking' sort of cough as you have described. A full physical exam and temp check will help rule this in or out. Again for more info please visit: http://www.marvistavet.com/html/kennel_cough.html
Furthermore, if you are taking your girl for walks, please be sure to use a harness rather than a collar or choker chain to ensure there isn't any pressure put on her trachea.
If you have any further questions, please feel free to reply! I hope this has been helpful for your.
Thank you and please now click ACCEPT.
Dr M D Edwards