I was helping a friend move this past weekend and his truck literally sounded like a diesel. He thinks it is a 2007 or 2008, it is a Ford F150 with the 5.4 V8. It has around 112k miles. He has not had it very long. It had a very distinct diesel sound, almost exactly like a newer 6.7L PowerStroke or newer Duramax. What can cause this in these engines? I don't know much about them besides the spark plug issue. It didn't sound like an exhaust leak, but internal engine noise.