When the motor spins around the brushes that allow current to travel to the rotor mechanism have their contact with the rotor start and stop very quickly thousands of times per second.
This causes the voltage potential across these contacts to vary wildly and combine this with the EMF (Electro Magnetic Field) and reverse EMF that is generated (when the coils lose power they spit back some current in the opposite direction), it can make for a serious amount of noise.
This is why in a lot of older cars (1:1 ones
) there is a bypass capacitor (AKA auto capacitor) across the auto transformer. This eliminates a lot of noise in the 12V supply line in the car so you can actually hear the stations on your AM radio and not just the pop/hiss/crackle that is EMF noise.
FM radios aren't effected by this only AM radios are. The transmission scheme used by these Bit Char-G is AM, hence why I believe that big black electrolytic capacitor is there.
For anyone not knowing what BillyP and I are talking about it's the black POS here:
ph2t.