Looking for some help... over the last few weeks our astra has started making a knocking sound from the back, at first when going over the lieks of speed bumps but now even when going over some rough bits on the road.

I'm guessing its suspension related, as I have checked the exhaust and it doesn't appear to be able to hit off anything. Also the springs look intact, so is there a definitive way for me - a non mechanic - to work out whether it's bushes or shocks?