take a guess at odd engine noise?

I'd try the MMO first; if it does anything, it will do in pretty quickly. (I personally would start with a pint only.) The sticking lifter or lifter with a bad check valve are good possibilities and common causes for such noise.

Also, run your hands near and all around the exhaust manifolds on that side, trying to feel for any puffs of exhaust gases, being careful with the hot parts and belts. Exhaust leaks can make an almost tapping sound, and will often change with idle RPM.