My RAV4 developed a knocking/tapping sound when the engine was started after a 5 to 7 hour break and the engine was cold. The noise would diminish or disappear after the engine reached operating temperature. I was told from the dealership that it was due to carbon buildup on the intake valves. They suggested that the problem was the result of my driving style, but they went ahead and cleaned the valves using a top engine cleaner. They also told me to make it a habit to drive on the highway more often. Apparently I have to babysit this car. I thought that port injected cars would be less susceptible to carbon build up.