It was a strange noise that followed wheel speed and would occur after hitting a bump. Turns out it was a rear brake pad that slipped out of place and the metal backing was dragging on the rotor. It wore a nice, deep groove in the rotor. The root cause was whoever did the previous brake job failed to install the hold down clips in properly (allowing the pad to vibrate out of place). Buth rear rotors need replacing anyway, so it's not that big of a deal.

Mark G