I currently own a 2000 xlt 5 litre AWD and have having some problems with valve noise. Rattles like crazy on 87 not so bad on 89 octane. Been using synthetic oil since I owned it. Has 167k miles. Other than that no problems. Tried the water injection method of removing carbon from the cylinders and that helped a little but to no avail. Marvel mystery oil helped a little. Could the lifters be worn? Or does this something more serious. Usually happens under accerlation and when its cold. Doesent sould like piston skirt wear as it makes the noise hot or cold. Thanks in advance .