That sound hurts me........ Way too loud to be normal OHC sound. I would bet it's rod knock from a spun bearing. I had the same tappety diesel noise when it happened to me. Did you check the oil level? Mine occurred just shy of 5k miles short of my oil change. The oil was low and I believe a hard turn did it in.
Here's how to check for a spun (obliterated) bearing
Remove all spark plugs. Mark a soda straw with a bunch of tick marks with a marker. Stick the straw in spark plug #1 (or, if you're positive it's #6, you can start there).
Turn the crankshaft nut with a 19 or 21mm socket CLOCKWISE as you are facing the engine.
When the straw stops moving outward, you've reached Top Dead Center. Turn the crank another 1/8 turn or so. The straw should smoothly move down.
Follow the 3.9's firing order of 1-5-4-2-6-3-7-8 for fastest results (only 1/4 turn between TDCs)
If you find a cylinder that seems to have a delay and then a jolt, you've probably found the spun bearing.
Stick a long screwdriver into the sparkplug hole and push downward, parallel to the sparkplug whole. If you've found your spun bearing, the cylinder will move down a tiny bit from the screwdriver
Cry
There are no rebuild parts (besides some racing Jaguar parts for several thousand dollars), the Ford-supplied reman is over $5k, and Jasper won't touch the LS (except apparently they'll sell you a diff for $1.5k). So now you can look at a junkyard, most of which have no idea how many miles are on the engine, eBay, which is significantly more expensive but not awful, selling it for less than $2k, or junking it