Rattling noise during engine start up.. 2005 mitz galant 2.4l

Probably a heat shield. Often time the screws that hold the shields in place rust away causing the shield to rattle. After the exhaust heats up, the pipes and shield expand causing the shield to stop rattling until it cools down again later when the car has been sitting for a while. Usually you'll find shields under or around the catalytic converter and around the exhaust pipe nearest to the engine. Most folks would probably recommend you remove the shield and be done with it. That's your call. Just remember though, the shields are there for a reason. In particular, the shield under the catalytic converter is there to prevent the possibility of starting tall grass on fire in the event you had to pull off the road for example to fix a flat tire or something along those lines. It might be unlikely, but its definitely been known to happen. Let us know how it goes. Thanks.

You probably have a bad plug wire or cracked cap/roter , this can be seen at night in full darkness to pinpoint it. or you can buy a mechanics stethoscope (about 15.00 or less) and listen for it.