Generally you would only hear timing chain rattle at start up, until oil pressure pushed out tensioners, and then at mid-RPMs
Good read here: Timing chain rattle resolution process - SOHC V6 - Ford Explorer and Ranger Forums "Serious Explorations"®
Explorers got the 4.0l SOHC engine in '97, Rangers got it in '01, so those forums have more early info on the problems.
Could the noise be Pinging/Knocking, which usually only happens under load?
Try running a tank of higher octane fuel, 91 or 93.
4.0l SOHC has 9.7:1 compression ratio so it WILL ping/knock on Regular gas, a Knock sensor is installed to change spark timing when knock is detected, if that is failing then you would get pinging/knocking under load with Regular gas.
Knock sensor is under the intake so under thermostat housing and it's wire comes out top front passenger side of valve cover and has a connection to main harness there
Picture of that connector here, post #9, lower left, yellow wire: http://www.explorerforum.com/forums/...d.php?t=351576
Those wires can get loose or frayed
Moving things around to get t-stat housing out and new one in could have weakened connection here.
Location of knock sensor seen here: http://www.explorerforum.com/forums/...d.php?t=353594
And here, 4.0l diagram: http://www.autozone.com/repairinfo/r...96b43f8036de4d