Superb 3T4/ 3T5 rear diff whining

Good day, i am driving a Skoda Superb 3T5 for almost a year now. Since the very first drive on the highway(autobahn) I've noticed that there is a whining sound coming from the rear end. My vehicle has this 4x4 option equipped with Haldex 4 clutch that was first time taken care of around 80-90k km(now i am at 114k km). The sound is only heard at 5th and 6th speed(it's a manual/ stick shift gearbox) which i find very odd. What might be the culprit? Rear diff had it's oil changed twice in it's lifetime