I had a stroke inn'93cand slmost every tme i hear a cough or loud noise my affected left armmjerks. why?

I don't have the information available about your stroke to say absolutely, but I will tell you that rarely certain types of movement disorders appear around the time of stroke or down the line. Myoclonus, ballism or chorea. Be prepared with a clear description or these abnormal involuntary movements, and talk to your neurologist. Good luck

Possibly a myoclonic jerk secondary to an exaggerated startle response associated with the prior location of your stroke. If this is too overwhelming, your neurologist could provide medication to prevent these events.