Thanks for response. Your description of the pump pressurizing the tank / purge system over-pressurizing the system sound exactly like what is happening.To everyone else - thanks for the laughs!

After driving the Vibe for a few minutes, I heard a huge noise like metal on concrete in the rear. It happened once every time I drove it.I took it to the dealer, they heard the noise. The gave me a rental becuase they couldn't find the cause. They had a guy lay in the back to idenify source. They p...