I have a 2006 Mercury Mountaineer AWD with the 4.6. At start-up, when the engine is cold, there is a loud and very noticeable knocking sound coming from the passenger side of the engine. I have ruled out the ever noisy injectors. After about 30 seconds, the noise reduces to an acceptable level. Lately, it is getting worse. I have chased the noise down to the front of the passenger side valve cover/exhaust manifold area. As I said, the noise gets better when the engine warms up. I am using the 5W20 oil recommended and change the oil before the computer tells me to. Any ideas as to what this could be?