Put the body on axle stands and check the suspension.
Start with the state of the rear subframe, check it's securely bolted down and look closely at the bushes for the transverse arms.
Grab each wheel at 12/6 o'clock and then 3/9 o'clock and rock them to check for movement. If there is any look more closely at the transverse bushes when underneath the car with someone else moving the wheel.
Check the outer bush on the hub carrier to anti roll bar joint.
Inspect the springs for cracks and the spring seating rubbers in case they're collapsed and you're getting metal to metal contact. Check the state of the top mounts too, not just the securing nuts but has the rubber perished?
Have a look at the state of the bushes in the brake compensator linkage to ARB. I'd look at the ones in the bracket bolted to the ARB and the ones in the actuator arm, Mark sells these, mine were AWOL. Finally have a look at the brake compensator itself, is the piston free or seized? If it's seized you may be hearing the actuator arm knocking against it on compression. Potentially, a road test with the compensator disconnected from the ARB over a pothole known to cause the noise will give you a good idea if that's the culprit. But be aware that your rear brakes may be more effective than before. Take it easy!
As with anything underbody related please make sure the car is properly supported before you go underneath it with a fall back support like a couple of carefully placed spare wheels. Take extra care if you're dismantling shock absorbers. Springs can be lethal.
