No codes, will not accelerate with throttle position and will hear a rattle also, air intake clean, and all data values on scan tool appears normal. Not sure if trans problem or what, does drive ok with normal driving, but with power demand, just stumbles and not a misfire problem looking at scan values.

Answer: If it was a transmission problem, then the engine would rev real high when accelerating but your Pontiac would not move. This does not seem to be what you are talking about.

You are describing the engine wont rev up. Check your fuel pressure. If that is OK, you might have a bad Mass Air Flow (MAF) sensor. These can act like a low fuel pressure condition. Another possibility, if this problem only happens on hard acceleration, is that you have a plugged catalytic convertor. This would not show up on the scan tool. Basically the engine cannot breathe. You would need to do an exhaust backpressrue test to diagnose that, but it is quite common.

Another way to test for a plugged catalytic converter is to disconnect the exhaust before the converter then see if the engine revs up.then the converter is plugged because now it runs fine.