To come back on my previous post. So putting a ceramic 100nF capacitor on the DC motor solved the interference (noise) issues. I did a lot of reading on the internet. Google was indeed my friend. I had no clue what size of capacitor to use but I ready few articles where 100nF was proposed which was the size I tested and solved my issue. The circuit is 5 Volt. Since the capacitor solved my issue I did not pay attention to test other values. Probably other values would have worked too. I have no electronis background so I cannot explain. I try to believe what others say and try it out myself.
In the meanwhile I have put together my remote car circuit (2 DC motors) and tried my application and it worked. I had completely disassembled the car circuit to the bear minimum in my investigations.
Well thank god that has been resolved, now I can have a look on further improvements, especially on my code.