knock in top-end

We have a 98 Explorer Eddie Baurer 4.0 SOHC, bought used with about 136k on it, the other day it started a knocking sound that seems to be coming from the top-end, "I think". It doesn't really sound like a lower-end bearing sound. Is there an issue with valves that could be causing this since it is a Over Head Cam engine? It does sound different than my 00 Ranger that has the 4.0 OHV engine.
Any suggestions would greatly be appreciated..