I just ran it for several minutes, and the sound seems to be coming from both the top side, around the valves, and from the bottom side, right around the middle of the oil pan. It's more prounounced on the bottom side, but when I pulled the oil cap and watched the valves, the timing seemed to match them. Sticking lifters/rods maybe?

97 TJ Sahara 4.0, K&N FIPK, 2.5" rancho, 31x10.5 BFG, black rims, tuffy stuff, hi-lift, AA SYE+tom woods CV rear, tera adj. upper rears, centerforce dual-friction, MSD coil/8.5mm, borla cat-back, warn steering skid, slotted rotors+high perf pads.