The worst case scenario would be a scored front main bearing or cam bearing. Either one of those gets into major rebuild territory. If something looks shiny, like the inside of the timing cover, it's possible a little sheet metal massaging might cure the noise. While things are apart it would be a good idea to look at the clearance on the rod and main bearings. Good luck!
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!