ford f150 with engine knock

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??

Sounds like you need to examine the bottom end. I would hold suspect the number six rod. When you remove the plug wire, no combustion softens the action and therefore the noise disappears. Makes sense ot me.

I'll have to agree with Typhoon. If you have no force on the cylinder, you have no pressure on the bearings. Once you apply the firing, its fighting back the compression and the expansion of the firing. Sounds like your gonna have some fun.