I hate to dig up a 2 year old thread, but I had a similar noise seemingly coming from the driver's side door panel (2017 Volt). I thought maybe something was inside the panel making the noise (sounded like a small ping-pong ball bouncing, or plastic sheathing bending). Then I noticed the noise continued after I stopped the car at home as I was getting out. It felt like it might be the seat. Turned out to be the little round plastic cap that is near the joint where the seat back and the seat bottom meet, right next to the B pillar. I had to move the seat back as far as it would go to remove the cap. Took it off, and drove around with no noise. Just for reference in case anyone else comes across it.