For some reason my 2001 4.6L V8 truck is knocking randomly when I start it. It will quickly knock then stop. Randomly a short time later it will do the same thing over again. I do not hear this when I am driving and it doesnt happen as much once it's warm. The truck has 213,000 miles on it right now, oil changed every single 3000 miles. Thank you for any help before I just bite the bullet and get a mechanic to look at it.