Different scenes can exhibit different noise levels even at low ISO's. Your example seems like a dark image anyways so one way to maybe improve on the noise there would be to slightly over expose and then back it down with your software. That usually would mean shooting at an even higher ISO but dark scenes just create noise. When I shoot with my 5D2 in a dark room at ISO 200 at slow shutter speeds I can really start to see noise as well. Its just a fact. If your camera is under warranty then I'd send it in if your really concerned. Most likely they will send it back to you and say they looked at it and even tweaked it but you will see the same results once you get it back. Its your call. If you process it in DPP do you have any level of noise reduction selected? What's the noise reduction setting that you have set in camera?