Does it go away after some times or if you change exhaust modes? Mine has a very similar reoccurring sound and it is from the exhaust valves and goes away after a few mins. It can be fixed with the TSB 22-2388 if so. The big thing here when I had mine done was the PCM (or BCM) update that came with the TSB since I guess that controls the valve position. I only learned this because after I flashed my tune, it seemed some stuff was changed in that same module and the rattle comes back and I don't have the time or energy to reflash the modules to get rid of a rattle on cold start after a tune flash.