2003 Ford explorer won't start, loud knocking?

Im asking this for my dad. His truck drove fine after work, he made a quick stop at a convient store and when he to back in his truck wouldn't start but makes a VERY loud knocking noise and then now it does turn over at all. He had no warning signals on his dash, no smoke from engine, or anything. He had the starter checked and thats fine and he just put in a new battery 2 months ago or so. His motor also still turns and has compression. Any ides what it might be for us to try before just putting it in a shop.

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