I have a 2001 Ford Expedition with a 5.4 L engine. I hear a whining noise from the rear end and would like your opinion on what might be the cause. The noise appears when I'm coasting. During acceleration or while maintaining speed, there's no noise that I can hear. Once I remove my foot from the accelerator, the whining noise begins, and it slows down in direct proportion to the speed of the vehicle. I changed the gear oil and replaced it with synthetic oil plus one tube of friction additive. What could be causing this noise? I need your experienced advice.