Wow, I've never thought of this as synesthesia, or really given it much thought at all, but I think I exhibit the visual motion to sound symptom sometimes. It's rare, seemed to be more common during high school years (I'm 21 now). If it was really quiet and usually when I was relaxed and alone, it was almost as if the ambiance would get progressively louder. I'm not sure if it was associated with visual motion or repositioning my eyes, but I would also hear whooshing sounds. It seemed like I could also hear faint sounds louder, like typing or the creaking of a chair. At times it would actually get annoying, but it would go away when I went about and did something to take my mind off of it. Since the beginning of the year, I can only remember it happening once, for about half an hour .