I just finished taking off the rotor and see nothing trapped in there. No pebble or small rocks fell on the floor. The sound definitely sounds like something hitting the shield but can't see what.
I adjusted the parking brake shoes one click in reverse just in case it was that.
I put everything back and tool it for a spin around town and didn't hear the noise again however it comes and go so I guess I'll need more seat time to see if it comes back.