My 2013 Energi also makes a loud-ish sound. almost a bearing howl, from the back of the car, behind the driver, when the gas engine is running; it has been diagnosed on the late lamented Energi sibling of this forum as a noisy fuel pump, and a fairly common issue. Seems to be harmless. Certainly, it has caused no trouble in the 6 years I've owned the car, including some multi-day road trips.