could very likely be the drop links. you can reach them with the wheels on, sitting on the deck. try disconnecting them, drive over some bumps and if the knocks gone, it's them. They are £36 each from EB Spares for example, but I got a pair for £22 inc del from buypartsby !! OK, there's no box with Alfa Romeo written on it but for that price ... Just checked the site - perhaps it was an error by them, they are now £14.95 + VAT each.
I had ordered some drop links already for 17 quid the pair. Just thought that while working on it I may as well change the ARB bushes. I guess if they arent available I cant...
Do these first and see if it goes away