Or piston slap. Does it seem to go away when the Jeep is warm up?

 

Is it in the front of the engine? Loose timing chain slapping the cover maybe?

 

Mine has a slight knocking sound, but only at idle and typically gets louder the warmer it gets. I thought it was the flex plate bolts but they were fine. I didn't have time to check for cracks though. Any ideas?
I'll pick up a stethoscope from HF when I get a chance.