1999 pontiac grand prix gt

We owned a 99 GrandAm GT with the 6 cylinder. They are prone to lower intake gasket problems. Had to have it replaced @

75K for $750. This would cause your sludge and could lead to valve problems. Are you losing fluid? How's the oil look?

there's still oil in the car. i had the radiator cleaned out and the shop told me they couldn't find anything wrong with it. but now there's that tapping sound...

A leaking intake manifold gasket could introduce coolant into the oil or oil into the coolant. Was your coolant level low? Did you notice any coolant in the oil (i.e. frothy appearance or bubbles on the dipstick)?

Hopefully neither is the case and the tapping is not resulting from damage done by a defective intake or head gasket.

Without hearing it, it is difficult to diagnose a "tapping" noise. It could be a valve or worse a bearing slap.

Best to take it to a mechanic and have them listen to it.