There is a bulletin on this. My '01 did the same thing. I took it in to the dealership. It was a design flaw in these engines. When the engine is cold, the pistons are contracting much more than the cylinders are. The rings cannot compensate for howmuch they are shrinking. Newer motors have shorter wider pistons for less friction and thus beter fuel economy. So when the pistons are cold, they are rocking slightly, this is called piston slap. Dealership said it ws a flaw but just let the motor warm up and dont place a laod on the engine till it does. I went to another dealership and they said the same exact thing but they pulled the motor put and replaced all the pistons with high performance polymer coated pistons. runs great and no more knocking or tapping sounds.