I believe I have a problem regarding engine idling. A few times when my wife comes home backing the car into the car port, the kitchen windows rattle. I pop the hood and check the engine idling and it's shaking like a leaf. On other occasions it's somewhat OK. Slight vibration, but not as bad. When we first got the vehicle the engine idling was steady as a rock. Does anyone have an idea what might be causing this before I take it in to the dealer and get a "nothing wrong here" answer?