Ford Falcon: Knocking sound

At around 90,000 km my BF Falcon XR6 developed an extremely annoying, but intermittent knock, sounding similar to something hitting the bottom of the car at the rear. The Ford dealer first replaced the outer diff bushes, but when the problem persisted they were at a loss to explain it. All they could do was to tell me the car was not about to break down. I had Pedders do a full suspension check and they could find no fault with the suspension either. But I have noticed other Falcons with the same noise. My car has not missed a service and has been extremely well maintained, but the knock is getting worse. Can you shed any light on it?