Hello from Kazakhstan. 1st of all, sorry for my english.
I've bought 2019 RAM REBEL at Dec,18. I hear this "startup-knocking" from the beginning of RAM use. Now it has 4000km of mileage.
My observations suggest that:
1. The sound happens at the start when PRESSING THE BRAKE PEDAL ONLY.
2. With a remote start there is no sound.
I found the following solution for this feature and I'm using this way and reccomend it for you all:
If you want to start engine by push-start and don't want to hear this unpleasant sound:
WHEN STARTING, DON'T DEPRESS THE BRAKE PEDAL STRONGLY - JUST TOUCHING IT LIGHTLY. (IF THE VEHICLE STANDS UNDER THE STAFF, ENSURE THAT THE PARKING BRAKE IS ACTIVATED)
I have not heard this sound for 2 weeks when I start the engine with a lightly pressed brake pedal.
p.s.:
My answer your early question:
The reason that the sound happens 2or3or4 times of 10 is only that these times you are harder on the brake pedal. that's all!
GOOD LUCK TO ALL!