I agree. Sounds like a Lower Intake Manifold Gasket to me.
I would just take it apart and once the LIM is off, check the coolant ports for signs of leaks past the gasket into the lifter valley.
If the LIM gaskets are original, they are probably WAY overdue. Fel-Pro makes an aluminum framed replacement set that does a good job. Also replace the LIM bolts. Fel-pro also makes a new set of bolts that will come with the right amount of threadlocker on them.