Vauxhall Astra H estate - 07 1.3 Intermittent high-pitched screech - No FM2R
I can think of nothing moving that could make such a noise at the back of a front wheel drive car other than the brakes and associated kit, and the wheel bearing.
It's not pure coincidence about the being warmed up and something like a handbrake cable touching where it shouldn't after a few bumps, I suppose?