I have a 2002 dodge intrepid 2.7 when i first start it it has a loud rattle for a second, almost like a rod knock. then it clears up and sounds fine. when i rev it up a little, this knocking sound appears and stays. sounds like its coming from the drivers side top end. almost like something is slapping. does anybody have any good ideas of what it could be before i tear into the top end of the motor so i know what to look for.? thanks.