Originally posted by: Captain Howdy
Gonna go out on a limb and guess you have rear drum brakes. It is a normal sound as the shoes just get past the point of not biting anymore. I wouldn't exactly call it a "croaking" but have heard the sound on any rear drum vehicle I have ever driven, especially on more humid days.
If you have rear discs, then it could be corrosion in the anti-vibration shims.