Start Up high RPM sound then diminishes

the reason why it starts up right away after you have been driving it is because the motor and systems that run it are good and warm. fuel has to travel from gas tank to the block in order to be ignited to thrust the piston, and it 's already there. you have a high rev, because the motor is cold and systems that run it. so therefore, if it makes any sense, the car revs high and then once the fuel strategies are figured by the computer, it calms down. I have a 4 x 4 96 Ford 5.8 motor and it does the same exact thing. No need to worry

Hello, My Nissan Altima 2013 at the time of starts RPM goes to about 1500 then calms down to 800 and stays normal. It happens usually in the morning after being idle for 3-10 hours. So for the first time in the day or if I start after keeping idle for more than around 3 hours. So I guess it’s normal to balance everything systematically. This is for your information and feedback to make sure mine is also running well. Thanks Gayan.