my mirror did the same thing. theres two options. 1st option is a mirror delete(lol jk)
2nd more logical option is take the mirror off and I folded up some bubble wrap and stuck it between the mirror & door. got the rattle to stop w/o seeing the bubble wrap. sounds ghetto but it worked and im the only one that knows its there.
05 TL Mustang Base (the silverskoundrel) - sold
05 TL Mustang GT - sold
98 UA Mustang GT - sold
10 CI Mustang GT - slow