I've been having a faint (but at the same time pretty noticeable) "wooping" sound like a helicopter coming from what sounds to be the rear of the car. When stopped there is no sound. When I accelerate though, it gradually gets more noticeable and seems to be in sync with the rotation of the tires, and when I decelerate, the "rotational" sound decreases in sync with the car slowing down. I had the rear tires replaced when the tread was getting low thinking maybe it was the tires, but the sound is still there. Any ideas?