Does it have an aftermarket panhard rod fitted? It may have come loose somewhere, that causes a knocking noise. Another thing (but more unlikely) - look underneath the rear axle by getting under the car. You should see a metal round bar running along the axle (inside the axle), see if that's broken anywhere. That broke on an old GT I had and it causes a horrible noise, took a while to find out what the problem was though! As I said though, unlikely it's that but it's worth a try if everything else is ok.