Ok, so just got back from a weeklong trip, and took the mustang out again. Had the window rolled down, and noticed a strange sound every few seconds (in every gear, at fast and slow speeds).
It sounds like its coming from inside the drivers side wheel well, behind the tire, almost like a quick half-second hiss of high pressure air releasing, or maybe a thin tree branch.
Its definately not every wheel rotation, or at gear shifts, so I'm scratching my head. Computer reads clean, I just scanned it.