New 450h owner here. Whenever I am at stop and my A/C is on, after a while there will be a Thud, and the car will shake once or twice, want to know if that is normal.

 

that's your engine starting. you can search and read more. if you are going slow on local, engine might shut off and just rely on electric engine running the car. but if battery is low to certain level, gas engine will start running to provide power