I have a 94 F150 that has started to clatter after about 30 seconds to 1 minute of running when the engine has warmed up. It doesn't do it when you first start it up. It almost sounds like the lifting rods clattering in an old Monte Carlo that I used to own. It doesn't appear to affect the performance any but it is very annoying and I want to ensure that nothing is major wrong. I can't seem to find anyone that has a clue about it. Any suggestions?