Rear Diff noise on my 2007 E82 128i - HELP!

I've got a rear diff noise on my daily driver, a 2007 E82 128i. It's been making a constant whining/humming sound when I'm cruising at moderate speeds or decelerating. Been driving like this for weeks now and it's getting annoying. Tried to diagnose the issue myself but couldn't pinpoint the problem. Help me figure out what's going on!