High Idling on a new car

Completely normal. It will idle fast until it starts to sense a little heat, the colder out it is the longer it will do it for. Its usually best to wait until the idle has come down before you put it in gear and start to drive. I personally start my car and let it idle for at least a min (to warm up the internal parts) in the summer before i start driving, in the winter i like to see the temp gauge start to rise before i go. Hope this helps

Reading you question again, you should make sure theres the proper amount of oil in it, really the engine shouldnt rattle on starup

Take that puppy back to the dealer and complain. New vehicles don't act that way. At all.

Owner's manual says it's normal and required to start up the catalytic converter. Timing is also retarded which gives it the weird sound. Manual also says that you don't have to wait to put it in gear and drive. "The Mazda3 is equipped with SKYACTIV combustion technology. This technology includes AWS (Accelerated Warm-up System) for optimal emission performance during cold starting conditions. ... What you will notice? On start-up, the engine speed and sound are higher (1500 RPM) for less than 1 minute to quickly bring the catalyst to efficient operating temperature. The RPM will reduce to idle once the temperature is achieved. This is normal engine operation under cold start conditions. ... NOTE: You can drive the vehicle during AWS activation, no need to wait."

That’s what the direct injection skyactiv motors do. I have a 2018 Mazda 3. I actually hate this car. Mazda is a disgrace to Japanese engineering. Mine has an idle problem at 33000 miles. Fml