I think what you're hearing is probably normal. The engine in the Volt is not as smooth as any number of small Japanese or European four cylinders, not even as smooth as the 3 cyl in my smart car. And in certain situations, apart from a steady highway cruise for instance, GM has not done a good job with NVH isolation. As long as it is not excessively loud all the time I wouldn't worry. Driving in electric mode only makes ICE noises more obvious.