I have a 2015 Volt that developed a really irritating squeak when the car was being driven over ANY bumpiness in the road at all.
After troubleshooting for weeks, I finally found the cause. There is center plastic piece that is between the two back seats, and goes about 2 feet forward towards the plastic piece between the two front passenger seats. It can be removed by unscrewing 3 torx screws towards the back of the piece, and 2 press-in clips towards the front of the piece. The 3 torx screws screw in to small metal anchors that are pressed in to a thick metal bracket attached to the body of the car. These metal anchors can slide with respect to the metal bracket. I put some teflon grease around the anchors so that when they slide, they don't squeak.
Now I'm enjoying the sound of silence.
Hope it can help y'all!