Hi my 1998 2.8 has a slight tapperty noise on tick over from the rocker cover sounds like a noisy tappet just done a engine flush and oil change but it hasn't made a difference how easy is it to set the tapers on these ? I guess they are hydraulic , is this a common fault ? As soon as the revs rise above 1000 rpm it sounds as sweet as a nut , any thoughts? Thanks in advance