Here's a thread with some info on that:
http://www.explorerforum.com/forums/archive/index.php/t-28607.html

Excerpts:
"Ford Motor Company will extend the engine warranty on all 1997 and
1998 Ford Explorer and Mercury Mountaineer vehicles built with a 4.0L
SOHC engine for the front cam chain guide assembly.

The additional coverage program will be in effect for 7 years from
vehicle warranty start date or 100,000 miles, whichever occurs first.
(so too late at this point.)

On some of the affected vehicles, the front cam chain guide assembly
may wear or break and cause excessive cam chain noise that does not go
away after five minutes of engine operation.

Dealers are instructed to install a new cam chain guide assembly if the
original is found to be worn or broken. If it has broken and fragments
have entered the engine oil pan, dealers are authorized to clean the
block of all debris, replace the engine oil pick-up and filter
assembly, and install new oil and filter.

By what I was told, if you come in for one of the three (sticking
throttle body, tensioners or intake gaskets) they are supposed to do
all three. They replaced my tensioners, which got rid of that god awful
noise at startup, but if ya pop the hood (which my gas tubes are
failing on, nearly lost some fingers) you can still hear the chain
running across the guides, freakin bikes dont make that much chain
noise."

-Paul