The spring can hit the rear board under compression if it's soft. Hard to check but if this is the case, there will be a witness mark on top of the leaf spring.
If that is the cause you'll need to fit a pair of springs. But first if you're still on original dampers, try some new adjustables. A slightly stiffer damper might stop the knock.
But the front springs can bottom out on cornering if there's a bump in mid corner. Can you sense where the noise is coming from?
