Jimmy, Are the pieces cast? Sounds like sponge or shrink spot
porosity, usually generated by cooling/setting problems-- too quick
or misdirected-- after casting.
The appearance of these areas is often worsened by the the
ultrasonic, which finds the pinpoint porosities and scours them.
Loupe the bad areas and look for a spongey or lightly pitted surface.
Good luck, Andy Cooperman