By rear, if you mean the side away from you, near the passenger firewall, it may be a cracked exhaust manifold. It's common to crack at the #6 runner. It could also be a fuel injector. Use a piece of heater hose to isolate the noise. Use it like a stethoscope with one end to the ear and the other as your explorer end.
To check the rocker arms near the trunk side, a long screwdriver used in the same manner, placed against the top of the valve cover. Disclaimer: Just be sure to put the sharp end against the engine and the handle against your ear.