anti rattle shims ?

as im sure of the rear brakes on a mustang just kinda sit there and the are exposed to have anti rattle shims on the end of the brake pad where they sit in the bracket mine did'nt when i changed them and the reason i changed them is because the back brake squeeked really bad well i changed them and it stopped for 2 or 3 days no it is doning it again just wondering if the absense of the shims could be causeing it thanks sorry so long :D