So this is driving me nuts. I'm a frigging car mechanic, and I can't find my own problem.

This normally happens on card starts. SO maybe EGR?
Here is the problem. When I start up the car, Sometimes something under the upper intake rattles. It comes and goes, lasting between 30s and a couple minutes. It normally does it on cold starts, but I have also had it do it while warm.

It sounds like a bad PCV, but it isn't. Is there something else under the intake that runs on vacuum, or another pvc?

 

Nothing under the plenum(upper intake) should rattle. Maybe you're hearing noisy injectors? Maybe try a mechanics stethoscope and try to isolate exactly where it's coming from.

 

that's what I thought. But it's definitely under the intake, and nothing inside the motor.

 

Well if its the oil splash gaurd u will have to remove the intake to check. I read the TPI manifold splash guards were bolted/screws in. The carb intakes had a press fit rivit looking fastener. But if your bolts/screws are loose that tin oil splash guard would rattle pretty good.

 

Ah, never knew they had this. SO maybe it is inside the motor.