I just bought an 05 SCREW 5.4 4x4 with 107K miles on it. I didn't notice any engine noises when I bought it other than the normal "diesel" sound that most of these trucks make. However, after I had it for a couple of weeks, I noticed a knocking/slapping sound at barely above idle. And I mean barely above idle!!! You give it very much gas and it goes away completely!. Also, it only does it after the engine has reached normal operating temp. I took it to a local mechanic and he said it sounds like the timing chain slapping. He quoted me a very costly repair to replace the chains, tensioners, guides, and gears --> $1300. I changed the oil about a week ago and used Pennzoil 5w20 synthetic with a Motorcraft filter. Ironically, that is about the time I started noticing the noise. Just coincidence???