[transamkid]

i know some of you guys have jeeps on here, so heres my problem
my friend has a '99 jeep grand cheerokee. we have been taking care of it and all, oil changes and what not then one day it began to make a noise that sounded like a pushrod.
so we pull the valve cover off, and everything is kosher under there, nothing rattling around, so we figured it may be a valve. to fix that we read in his haynes manual to just put high viscosity oil in it *havent done yet* b/c oil was just changed and mobil 10w30 was put in.
so the other day he said he got home and it said add coolant so he puts some in, and it overheats and shuts off, probably due to the overheating, and before it shut off, the pushrod sounding noise was really bad (valve?)
what should we check, replace, what not
its a 4.0L inline 6, 2/4wd