OK So when it gets warm the problem is there.
therefore you have to look for parts that that heat up.
Exhaust hangers,exhaust joints,disc backing plate rubbing,
My guess is that the handbrake mechanisum where it goes into the drum.
take drum off remove brake dust etc a little grease on joints,you have brake shoes in the rear drum for handbrake.
When you first stop to park up can you still get sound when you push down on rear if you leave handrake off.