2002 Ford Explorer antifreeze out of tailpipe

My Explorer with 112k had a CEL "misfire in cylinder 6". I put in new plugs, and found that it had oil on the plugs 6 & 8. New plugs stopped the misfire but CEL came back on. Same code, (but still running fine). Took it to Firestone, and was told it needed an intake gasket kit. After leaving it for repair the manager (who examined the truck the day I brought it in) called and said after finishing the job it ran fine for about 5 mins then began to knock. He asked me did I ever have anything like this. So I went to the shop to find my truck with a loud knocking from the engine and antifreeze leaking from the tailpipe. It did not have any knocks, smoking, oil burning, cooling problems, and never overheated before. So you can imagine how upset I was to hear "he thinks it could be the head gasket (which seems strange because there is no white smoke) or previous problems"! The manager told me to wait and let his guy break the motor back down and check it out before we jump to conclusions. I seriously believe the mechanic is responsible. In your opinion what could have happened?