Whining noise from the rear when coasting, goes away when I accelerate

I've got a 2016 BMW X3 xDrive and I've noticed this high pitched whine coming from the back of the car when I let off the gas, especially between 60 and 80 km/h. When I press the accelerator, the noise stops immediately. Then when I coast again, it comes back. A friend who's a mechanic said it might be the rear differential, specifically the pinion bearing. I checked the fluid level and it was a bit low, so I topped it up but the noise is still there. I don't want to just ignore it until something breaks on the highway. I'm looking for a Proven Differential Repair Service that can actually diagnose whether it's the bearings or the gears themselves. Has anyone here had this exact symptom on an F25 X3 and found a fix that didn't require a full diff replacement? Also wondering if a worn driveshaft center bearing could make a similar noise. The car has 130k km on it and otherwise runs great. I'm in Abu Dhabi but willing to drive to Dubai for a specialist. Any recommendations for a shop that won't just quote me for a whole new differential would be great. Thanks.