I've noticed that this banging sound happens more frequently under colder outside temperature. It's coming from the bottom of the car when I press hard down on the gas pedal. Especially one day when it's only 10 degree F outside, I could hear it as soon as it reaches 10 mph. I don't hear it when the car is totally warmed up though. So I brought it back to the dealership, I left my car there for them to duplicate the sound, but they couldn't hear anything. Does anyone has any of this experience?