Thanks for reply..
No I start with the 1st gear and shift the gears one by one. When gear shift to the 3rd/4th, the car is moving at 40Km/h without pushing accelerator (I used the word "idle" for that scenario, and sorry for mislead) on the flat road.
I also noticed that even on the 1st/2nd gears with 20-30Km/h without pushing accelerator. But have to push accelerator to start moving from standstill to 1st gear. also no Knock and stop
So.. I think those information would be useful