I think it's the timing belt tension changing at low speed as the ecu is manipulating variables. There is variation in timing belt tightness, especially with age, leading to different people having different levels of sound. My wife and I both have 16's and both do it some. I need the hood up standing near the engine, but I can hear it clearly. Always thought it was normal. While it's a total guess, I just think the belt is making the noise. Think of all this little teeth whirring around behind a thin cover.