Sometimes, there is this noise which appears randomly when the car is stopped at a signal in D gear. It appears the rpm is quite low at this point, and the noise sounds like a belt noise. This noise disappears if you change the gear to N or P which raises the rpm slightly. The noise also disappears as soon as you accelerate slightly from stand still in D. If you switch on the air conditioning, the rpm rises and the noise disappears as well. The noise appears at random times, but always when the car is stationary in D. Any idea what this noise might be related to ? Alternator belt or tension? Or any other thing?