Put my 2008 S3 in for a Haldex service on Saturday morning with VW branch. On the drive home there was a loud buzz/whine coming from the rear. Then went away and now it's a steady whine, not as loud but still noticeable. Around 35-40 mph it happens when I very lightly touch the pedal. They changed it again today, still doing it. Going back tomorrow to have it looked at buy there master tech. Any ideas what this is ?