No wait!
I took the arms off the servo which were connected to the breaks and voilá! the sound disappeared! So, the sound came from the breaks, and when I roll the kit around now, it sounds a lot better. (The only thing I hear now is the sound of the breakdiscs touching the breakpillows + the sound that the tires produce)
But please stay with me if I have further questions! xD
Model: Team Losi LST1
The sound was coming from the brakes as they were half breaking