After the truck sits overnight, pull the drain plug carefully and see if water comes out before oil. How fast does your water level drop?
The knock could be an injector, but if it has had a lot of water in it you could have also wiped a bearing or more out. Headgasket failure on a stock truck is not very common, but anything can happen.
Darin 99 ranger 97F250 66 fairlanes
"TheBeast"1997F2504x4dually4.10's,160k,140VIDM,HPX,4-5"dp, S366 turbo w/wicked wheel, AFE, elec. fuel,rosewood stage ones,php hydra,arp studs, comp springs, smith brothers push rods, adrenaline hpop,riffraff res,melling lpop,180 tstat,6.0 fan and IC,trucool,coolant filter,scan gauge,lots of other gauges,racer x valve body and billet converter, upgraded trans, F550 leaf springs,SD dual tensioner upgrade, cold ac mod,OBA,38gal