I have been experiencing a knocking noise in the engine for 4 months, it only happens when the car is idle and the engine is warm. Its currently back in the dealership for its fourth visit but they have been unable to locate the problem. The noise 'knocks' for 4 beats, 4 times then stops for about 5 seconds then repeats the sequence until i start moving again. The vehicle is 15 months old and has had its annual service - Any ideas???