We've had the S for just shy of 6 months and have about 10K miles on it. I've noticed that when charging or sometimes waiting at a light or when the car is stopped that there's a "thunking" sound that is coming from somewhere in the car (seems to be from the back). It sounds like a "thunk" or like if you have a hard ball that is rolling around on carpet (if that makes sense). Is this a normal sound, it seems to be - perhaps - tied to the battery. BTW, we don't have anything rolling around in the car - and it happens when the car is completely motionless when charging.