When I shut the car off I get a rattle coming from what seems to be under the driver seat portion of the car? Is this just the heat shield because I have driven other z's with the same problem?

 

There is a heat shield under the car just rearward of the driver's seat. It'll make a metallic rattle if you thump the edge, but I've never heard it any other time.

You could try putting a piece of foam between it and the body to see if it's the culprit. Maybe putting a slight bend in the unsupported edge would keep it from rattling.

 

heat shield for cat or there is a possibility that the internals on the cat are coming loose. Those usually rattle continuously when the car is running though.

 

Sportsfan811,

Look underneath your car, specifically just aft of the muffler sub assembly. There should be an exhaust mounting bracket as well as a rubber component right next to it (it looks like a bushing):

Part Code #20722M for the bracket and 20651M for the bushing in this 350Z Exhaust system diagram

On my '03 Z, the rubber mounting bushing was ever so slightly out of alignment and this caused a rattle similar to the one that you have described. Inspect these parts and if any of them are out of alignment then re-align the parts and start up your Z to see if this resolved the issue

 

I know it's not the heat shield for the cats because I had test pipes on and it still made the sound. I read something like the 350z flywheel rattles, but not to worry about it does anyone know any more info about this?

 

So-called flywheel rattle is normal, but it happens whenever the car is idling in neutral with the clutch engaged (pedal up).