yup, that's your timing chain cover. I'm actually dealing with the exact same thing right now. I have it pulled off at the moment. Your chain is probably getting old and stretched, but it's not that big of a deal. I'm replacing my cover, chain and gear set. If it's making that sound, then you probably have metal shavings floating around in your oil.... Fix it quick and change your oil!
1990 YJ 4.2 5-speed: 4" spring/2" shackle lift, Rough country RS 9500 winch with synthetic line, Highlift and hood mount, WARN rear bumper, Custom front bumper, 33x12.5's (all five) on black rock crawler wheels, and crappy old speakers!