I had this done a few months back, they replace the centre section of the exhaust and mine doesnt have the problem anymore. You could always get an aftermarket exhaust and that will sort it too.
It is a very common problem, ask the RT to go in the car with you so he can hear it for himself, that is what I did.
I had it done for FREE under warranty. Dont let them rip you off by paying for it, especially if your car is fairly new.