2007 BMW 328xi automatic. I'm getting a whistling sound while decelerating. Happens when gas level is lower than 1/4 tank. The whistle/whining sound stops when even slightly tapping on accelerator. Also don't hear the noise when I add gas to the tank to bring it over 1/2 tank. Car has 73000 miles on it and changes gears smoothly with no other problems. Noticed slight lose in MPG as well.