98 5.4 Loosing Coolant

My 98 5.4 is loosing coolant. There is a milky white on under the oil fill cap. The coolant is not leaking externally. We put Dike in the overflow and it sealed the problem temperarily. Has any one had a problem with their intake or manifold leaking? Am told that it's poosible the intake is pitted and it will need to be welded. Help