[Generation "X" Dad]

I'm going to say it is the backing plate rubbing against the rotor. With the dust lip design of the rear rotor and how it mates with the respective backing plate lips, it does not have a lot of tolerance for buildup in those areas. The wheelbearing will flex a slight bit under side-load and allow a very minimal movement of the rotor towards or away from the backing plate and if you have rust buildup anywhere, even slightly, it will make noise.

When you remove the rotor, see if there are any signs or contact on the backing plate. Also, this would be a good time to wash your parking brake mechanism with brake cleaner and when it it dry, lubricate all points where the parking brake shoes contact the backing plate.

My '04 does this now and then as well.