The car has 80,000 miles and it has a whine from the rear end when you take your foot off the gas and coast. There is no noise when accelerating or light throttle cruising and there is very little noise when you take your foot off the gas at highway speed. The noise gets noticeable around 40 mph and is quite loud at 25 mph and less. Putting the transmission in neutral has little effect and neither does stepping on the brake. Is this likely a differential or ring and pinion backlash issue? Where do I start to diagnose the problem?