I'm putting this in Anything Goes because it's not related to Jack's amputation (which was the result of an injury), but if you think it's better in Ask A Vet, feel free to move it!

I'm at the end of my rope with a health issue and wondering if anyone here has thoughts on what could be causing a strange cough/gag that Jack has had since late April. What happens is she will stand up very suddenly and cough (which sounds like a long, raspy exhale) once, followed by a single gag once or twice a day. It's not brought on by anything except when she pulls on her harness - exercise/excitement otherwise don't induce it. It happens semi-regularly first thing in the morning when she's just woken up and other than that it's random, which is why I haven't been able to catch it on video. It hasn't gotten worse since it started, but it also hasn't gotten any better. 

We've been to the vet and she's stumped. It's not a collapsing trachea, nor does she have have heartworm or kennel cough. She doesn't have any other symptoms associated with heart disease from a physical examination. We've tried Benadryl, which hasn't made a difference, and have switched to a harness instead of a collar. She seems generally fine otherwise, though I sometimes think she's breathing loudly, but that may be a result of the heat we've had this summer. 

We're going to go back to the vet and I think x-rays are next (ugh...), but I'm wondering if anyone has heard of similar symptoms so I can prepare myself or other things to look out for or discuss with my vet.