are you on DIG?? When I was on that it made me cough. I think some of the nmeds can make you cough. And when I had a lower EF I coughed more.
I don't think it would have anything to do with the mitral valve. ACE inhibitors are notorious for causing a cough, but it wouldn't be only with exercise. If it is only then, then I wonder if you might have an exercised induced asthma, and that it would have nothing to do with the DCM. If you are well compensated enough to exercise, you must be in pretty good shape. If you had some decompensated CHF, I would think that a cough would be all day, esp. at night. It is worth mentioning to your cardiologist, though, in my opinion!
If you have cardiomyopathy, chances are you're taking an ACE-I. This can cause coughing. I would mention the symptom to your doctor though.