I’ve got a 2015 BMW 550i xDrive and over the past month or so I’ve noticed this faint but persistent whining noise coming from the back of the car, kind of like a low pitched humming sound that gets louder as I go faster. It doesn’t change when I turn left or right, and it’s not tied to engine RPM because if I shift into neutral while coasting, the noise stays exactly the same. A friend suggested it could be the rear differential starting to get unhappy, possibly just low or old fluid. The car has 140k km and I honestly don’t know if the diff fluid has ever been changed. I’ve been trying to find a shop that does accomplished differential care for BMWs because I know these cars can be picky about the exact fluid type. Has anyone here had a whining diff that was silenced by simply changing the fluid and adding the right friction modifier? Or am I looking at a rebuild in my near future? Really hoping for the cheap option since I just spent a bunch on other maintenance.
.
