I have already posted info regarding this annoying 4K rattle/buzzing. I spent about 20 hours trying to locate the source. It WAS the heatshield bolts that hold the middle and rear sections together. The bolts are too long and when done up, they actually touch the floor pan, but only just! I packed the bolts out with a small washer and cured the noise. This was sometime late last year and the customer has not been back! If I had not have cured it, believe me, he would have been on my case! Ask your dealer to check the bolts to make sure they are not touching the floor pan when tightened up.
Hope that helps
Bob RTE