95 Buick Lesabre 3.8

The engine has a tapping noise coming directly from the intake manifold. It sounded like it was a bad valve or a spring or a stuck lifter. I changed out the head, and lifters even though they appered alright. When I started it, it fired right up idoled good, but the tapping was still there. It doses'nt sound like it's anything lower end. It has a plastic fuel induction system on top of the intake, but there's no cracks or defomations. The engine runs good, except for the tapping. There's a horizonal shaft under the intake manifold that looks like an oil distributor the book dose'nt cover, it's on the top of the block; It's the only thing Ican think of that could be bad. I know this is kinda scetchy, what do ya think? Dumbfounded