Stuttering noise on electric only

I am slowly getting used to driving our used 2009 Prius, having used it now for just under a week (~175 km). For some time now I have noticed a slight stuttering noise while the car is on electric only. What happens is when I come home and go into our underground parking lot the car stays on electric power as my speed is less than 15 kph. As I back slowly into my spot (speedometer at 0) and the drivers side window open, I hear a slight sputtering sound from underneath. Could this be from component other than the electric engine as you know it makes very little sound and if so is more smooth.
TIA
Paul