I've noticed the child safety anchor covers are pretty loose and flap when I have all the windows rolled down. When they're rolled up, I can hear at least one of them vibrating. I haven't had a chance to secure them yet to confirm this is the buzz I'm hearing, but I'm pretty sure it is. I'd check that out on yours and see if that might be the cause.
Also, I've noticed sometimes the rear seat belts can vibrate causing noise. I don't seem to get that all the time, though.
On my old 2010 WRX I had a creak coming from the air vents on the dash that I never could resolve, luckily this hasn't come up yet on my STI (yet).