The rattle is coming from the connecting arm between the actuator and the valve. I have same issue on my 2016 M5 with M sport exhaust. I found the link below. This is exactly what is happening with my exhaust...and probably yours. You can touch the arm (wear a glove so you don't accidentally burn your hand) and stop the rattling noise. I'm working with Akrapovic now trying to get them to address issue via my local BMW service center (Classic BMW Plano, TX).
http://f10.m5post.com/forums/showthread.php?t=1285252