I have been done a lot of research and reading the last few days to figure out how I can solve my problem. Finally I ended up at the following page about DC Motors. See DC-Motors.
There is a part about noise caused by the DC motors. To isolate both circuits (Raspberry Pi linked/Battery holder linked (l298n->DC-motor)) I have to use an opto isolator (opto coupler). I did some further investigation and yes, that’s the way I wanna go. It explains the issue that I have. So I immediately decided to buy a couple of those opto isolators(4n35 FSC Optocoupler) on EBAY. I can’t wait to get to work but I’ll have to work delivery which will take a few weeks :-(. The opto couplers are on their way from China.