Horrible sound at the rear!

[Mr nanotech]

I'm getting very worried now. I backed my car into the drive way yesterday only to hear this horrible metallic groaning coming from the back.
It only makes it in reverse and pretty much only when backing up or down a slope. on flat ground it's fine.

I had Dad back the car into the driveway while I squatted behind to see if it was the exhaust scraping the ground or something. It sounds like it's definitely coming from somewhere behind the wheels. It sounds louder on the left rear than the right from what I can tell. I have no idea what it is.
I know this is pretty broad and it could be any number of things like shockers, bushes, seizing brake calipers etc... I haven't had a chance to take it apart and look around but I can say straight off the bad that when I lean on the rear of the car and push it down, It's quiet as a mouse.

It only makes this noise when I'm in reverse gear and backing up a slope like a driveway or something. It doesn't matter if you are backing in from the left or the right, it makes the same noise regardless.
To describe the sound, it definitely sounds metallic and it's a loud deep groan/grind/crunch noise. I'm not driving it still as I don't get my licence back for another two weeks, but just backing it into the driveway to get a trailer out, sounded like death.

Has anybody else experienced anything like this? Should I be worried? What is wrong with my car???

[Regie]

possibly the Diff..

it would have a fair bit of stress on it going backwards and uphill..something its not really designed to do

[manga_blue]

Small stone jammed between the brake rotor and its guard. Take the wheel off and feel for it with a screwdriver. Bend the guard back a little to ease it out.

[Old Dude]

+1 what Manga blue said, I had one in the front brake, it makes a terrible sound.
Cheers
Dale