Rear Axel Noise

Earlier today when driving over some speed bumps I heard a loud 'twang' noise coming from the back left wheel, almost as though a spring had snapped or something.

It only seems to make a noise when coming down off speed bumps. It doesn't make the noise when going over bumps on the road no matter how bad they may be.

Had a quick look and both springs seem to be whole so don't know what the noise is.

Anyone got any ideas? Any help would be much appreciated.

Going to have a closer look tomorrow.
 
Is the ride height the same on both sides? That will confirm if it's spring or not. The only other things to check are shock absorber top mounts and axle bushes