backfire when engine turns on while driving

When driving the Chevy volt in HYBRID model which means that your battery is dead and it will drive like a hybrid instead of a electric car, the engine will turn on and off, when this happens you will hear a LOUD pop from the rear of the car, this has been confirmed on video that I don't have by other Chevy owners to be a backfire, people walking near me when driving in a neighborhood can hear this pop! when in heavy traffic it is the worst!!! my commute is 100 miles a day so I always have to use my engine and this noise drives me nuts!!!!! I have brought it to the dealer and they tell me its normal and even had me drive another volt which had the same problem! this is not normal people! I have driven other hybrid cars and when there engine comes on all you hear is engine! also this noise doesn't always happen, if you cause the motor to turn on when it is parked you do not hear a pop and if it comes on with no demand like at a stop light it does not do the pop, GM has to fix this problem!