1997 Explorer 4.0 SOHC idle noise?

I have a 1997 Ford Explorer, with the 4.0 SOHC engine. The vehicle is 4 wheel drive if any are wondering. After turning over at a cold start, the vehicles starts fine, it revs to about 1300rpm, then back down to idle at around 700-750, after idling for about 5-10 seconds, it drops to around 525-550, and it begins to sputter, i can give it gas, and after it's warm it stops. Heres another question, at its normal idle, the engine makes it's typical noises, but when putting it in drive (putting the engine under load), I get a clanking/rattling from the lower crankcase, just above the oil pan. This same vehicle has a new engine, as the old one's timing chain tensioners went out. I'm hoping a Ford Technician can give me some ideas on might be wrong, cause ripping the old engine apart was a PITA! It doesn't quite sound like they're bad yet, but perhaps the tensioner is warn? Or maybe its something in the lower end (main bearings), its definitely distinguished to the lower end of the engine. Any ideas?