When I owned a VW EOS, it developed a slight grumbling noise at the back, along with uneven tyre wear, a bit like saw teeth. I changed the rear tyres and the noise disappeared, apparently it was a known issue and a characteristic of that car/platform. I don't know if the TT has a similar issue.The dealer has said it’s down to the rear tyres being stepped (uneven wear). They have changed the wheels around and rebalanced them. I need to take the car on a quietish road to see if it makes a difference, haven’t had a chance yet. This does seem to be the standard dealer response though. They have said if it hasn’t made a difference after 500 miles get back in touch.