Why does my 99 Ford explorer make a rattling noice when i get above 60 mph or going up a hill?

Why does my 99 Ford explorer make a rattling noice when i get above 60 mph or going up a hill?

Before you replace any engine components you should check the heat shield on the catalytic converter. They are notorious for rattling at certain times depending on how loose the shield is or how many of the spot welds have cracked.

What engine? The 4.0 SOHC has problems with the timing chain tensioners that can sound like a old diesel engine.

If it just started doing that, could be just a weak tank of gas. Fill it at a different brand station & see if it goes away.