i dont know what it is, but I can tell you that over the last year the noise has diminished significantly. Im assuming its something normal, as it happens every time predictably and ever since the car was bought. I wouldn't worry about it at all.
I have a '13 wrx hatch and I noticed the same noise but it only happens when it has been sitting for a long while... weirdly something else happened to me. I was driving my car after its been sitting for awhile. It made that weird noise as usual. After driving down a couple blocks I made a hard left turn to avoid a red light and that weird noise became a contant noise. Sounded like metal scraping. I continued to a gas station that was a block down filled the tank inspected the car (when I heard it I thought something under the car broke off and was dragging on the floor) everything looked fine.. got in the car and the noise was gone once I started moving. When I heard that sound I almost broke down in tears thought my baby didnt love me... but it has yet to happen again even when I purposely make hard turns just to see if it occurs again.