1999 F EXPL XLS 4.0 has chatter

Hi Cody ... depending on the mileage on your explorer since the belt has been replaced (i.e. if it's over 60K since) your friend has probably hit it on the head. You want to confirm or debunk that theory before the belt proves him right however because it's a much bigger fix after the belt lets go. Have a mechanic take a listen to it, especially if the "chatter" is internal. Good luck!

My 99 EXP XLS has 4.0 OHV not SOHC.  I am told SOHC is the engine with problems with tensioners and faulty cassettes.  Does the OHV 4.0 have any similar kits available b/c it has the exact same chatter?

I believe the belt tensioner issue is ONLY the OHC engine and not the OHV engine, although the timing belt could be bad on either. Could be valve train noise in any case in the older 4.0; best to let a mechanic look at it before jumping into any teardowns.