I have had my 2014 for about 5 months. I have heard the "acorn" a couple of times. That is exactly what it sounds like.The spoiler expanse sound is a "pop" sound like an acorn hit the back of the car. The gas tank pressurization system (in the right rear) is a more rhythmic sound, like a small buzzing/banging sound.
2014 Volt Premium - Safety pkg 1 and 2, Navigation