There could be something wrong in the computer controls causing a backfire (like a problem with the electronic timing control). Take it to the dealer immediately to avoid possible engine damage. Is the vehicle still under warranty?
Look for air leaks. Broken or deteriorating vacuum hoses, hissing noises from around the intake manifold and throttle body gasket. There should be NO openings to outside air anywhere from the airflow sensor to the engine.
A cross check of several sources shows that replacing the transmission fluid and filter every 30k miles is the manufacturer's recommended maintenance for your vehicle. Concerning independent mechanics, the answer is YES, they will be able to check/service the fluid as well.