I had a really annoying rattle around the passenger side somewhere, and after loads of fiddling around, tightening up a bolt on the end of the dash totally stopped it. Try it at both ends. I think these kinda adjust the way the whole dash sits slightly, so it basically tightens the whole dash unit, but do not take my word for it at all.
I've still got a slightly annoying rattle around the back of the car somewhere!