I need a hand with this one. I am mechanically inclined but not a wizard on motors. I have a 1993 ford F-150 with a straight 6. I am the 3rd owner and the truck has high mileage. It runs or use to run very well. I drive it only on occasions to my hunting lease so it is not my primary vehicle. 2 days ago when i started it up it began knocking very loud. After basic trouble shooting (checking fluid levels,condition of plugs etc.) I noticed that when the spark plug cable is removed from the last plug (closest to the firewall) it stops. I have asked a couple of mechanic buddies about it and all they can do is speculate. One says broken piston seal, one thinks a lifter needs adjustment (but it's way to loud), anther suspects a damaged fly wheel. Any Ideas??