Ford Repair: 5.4 L back fireing, coil pack, dual exhaust

Question
i had a spark plug blow out of my 5.4L engine in my 99 F-150 and had my machenic retape the hole and put the thread sleeve in and seal it in and its holding but i have true dual exhaust on the truck and can hear it back firing out the side the plug shot out when in idel. i put a new ignition coil pack in and it didnt change anything. Any ideas what it could be or any way to fix it? my mechanic has no ideas.

Answer
do you actually have a misfire at idle or do you just hear slight popping noise-a sliver of aluminum may have gotten hung between a valve and the head if it don't blow out on its on you may have to pull head-did you replace plug-

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