Knocking F150

I have a 2000 F150 automatic , V-6 with about 180,000 miles...it sounds like the valves are knocking, no real rhyme or reason as to when I hear this...even with High Test gas, but doesn't seem as bad. I've replaced the plugs with top of the line platinum and new wires but knocking started back after about 10 miles. Brought it back to the shop and found #2 cylinder plug was fouled out. Mainly hear it when starting off, but if I get on it, there is no problem or knocking. I change the oil every 5,000 with synthetic. Several mechanics have looked at it, but no solid conclusions...dang, I'm about to pull my hair out. Can anyone help me?:banghead:

Any smoke in the mornings or disappearing coolant? Sure sounds like a leaky intake gasket has very slightly bent a rod. Had it compression tested? Even if it is a minute bit it will show low on that cylinder.

No, no smoke and no noticeable loss of radiator fluid...but that's the most promising thought so far. Going to go see if I can get that checked today. Thanks,
Richard