I have an 03 hyundai elantra gls automatic tranny and when i am stopped at a red light, there is a knock in my engine. I shift into neutral and it goes away. When the light turns green and i start moving again, the knock goes away. My hyundai dealer mechanic says it's just normal engine knocking, but it really concerns me cuz i remember when she used to purr like a kitten. Does anyone have any idea what may be causing this and how I can have it corrected?