Sounds like a valve lifter has gone south to me. It might be best to pull the valve cover off and see if a lifter has collapsed or maybe been thrown out.
Hopefully this has not been going on long because one this noisy will damage a cam lobe, mushroom the valve stem, and so on. See if there is any excess movement in the rockers.
There might be some hope for one of the “renew-zit” additives to help but I would recommend using a stethoscope to investigate the source to determine how critical it is.
I does sound a little like a collapsed lifter, but this engine has a history with a knocking sound after start up that gets progressively longer until it knocks all the time. The knocking is usually from the crankshaft bearings. It starts around 70k miles and by 100k, the engine will need a complete rebuild.
BTW, it is identical to the Mitsubishi 3.5 DOHC V-6, but the Mitsu uses better parts. Some rebuilders will use the Mitsu parts when rebuilding.