A gear whine that follows road speed, louder on and off the throttle. Ring and pinion wear.
Worn ring and pinion gears or low gear oil in the differential.
Check the diff fluid level. A whine that changes between drive and coast is the classic pattern.
Reading about sounds only goes so far. The tuner on our home page listens in your browser and tells you what it hears. The app goes further: it names the exact fault, checks it against your vehicle, and tells you how urgent it is.