You need a new dealer.

 

If it's a rattling noise rather than clunking noise, it could be a harmonic. Something may be loose, and it only vibrates at a very specific RPM. On my Lotus it was a loose clamp on the muffler.

Have somebody rev the engine for you while you go to the back of the car and try to isolate exactly where the noise is coming from. Of course you can't just open up the engine compartment, but you could jack up the car, remove the diffuser, wheel well covers, and the underbody panels. Then with a mallet (or something other than your hand since some components are going to be hot), touch different elements (transmission casing, etc) and see if the rattle stops when you touch it.