Crack/spark sound when slow down in low speed

all,
If anyone experience similar problem?
I recently found my car has this crack or spark sound when I brake, especially in low speed; The sound comes from rear (I can't really pin down where it comes from), but I felt it like from the battery. It happens most when the car in the heavy traffic, when it moves slowly and stop frequently; if reduces speed from say, 60kmh or above, it won't have that sound when brakes.
So I think its more when the car runs electic motor.
Anybody has similar experience? or any idea what may happen (I haven't get chance to ask garage yet)

Many thanks,
Yanathome