No insight particular to the Volt, sorry. But this reminds me of my 1983 Volvo GL that developed a clunk sound going over certain types of bumps. Sound was coming from the firewall area on the driver's side. The dealer shop spent hours looking for the source. They pulled the front suspension apart. They finally gave up and told me to just drive it until something fails. So, I went home, got a flashlight, and started crawling around. It took me five minutes to find a dangling device. It was a small device into which the speedo cable screwed, that kept track of when the emissions system needed service, every 30k miles. It was supposed to be clipped onto the body sheet metal, but had come loose, and whenever I hit a certain type of bump, it would bang against the firewall.
I bet your rattle over bumps is something similar.