300C Noise

I took delivery of my 300C two weeks ago and have driven just over 350 miles so far. The car has developed a noise which seems to be coming from the right rear area sounds like a seat belt retractor being pulled in and out when accelerating or driving over uneven pavement. On smooth road no noise ! I can't find where this is coming from and thought perhaps some of you other owners may have experience the same problem. I haven't yet taken it back to the dealer for this.