First and most probable cause is the AR bar drop links... The longer you leave them, the more movement on the AR bar bushes...
Don't muck about with spanners, sockets etc... The chances are you will end up very frustrated trying to remove the old nuts after two hours.
Carefullly use an angle grinder with a cutting disc, You will have the old drop links off and the new ones on in a matter of minutes... They aren't expensive and a good place to start eliminating knocks...
Now... one of my bug bears
... Bloody AR bushes and the blase' regard to recommendation to "oh, just change them"
Err.. not as easy as it looks and the pay off simply isn't worth the effort or cost... If the drop links are doing their jobs then the AR bar is too... regardless of if it knocks, its a comfort issue on the ears
The simplest fix that I have found is to: Using a tube of Black Hardening type of Urethane Sealant (from general hardware stores like Jewsons etc.)and inject it into the gaps of the rubber bush, leave it to harden for 24 hrs (as long as you can really but 12hrs is acceptable), job done and you won't have to do it again.
To make it easier, try getting a hyperdermic syringe and needle for accurate application.
It hardens with the same density of a solid rubber engine mount gripping the bar!
Lets face it, if you ever need to replace the bar, you will have to cut the old bushes and clamps off anyway and replace them so worth doing IMO.
I once beefed up a S1 RS turbo bottom gearbox mount that kept failing with this stuff... was still going strong seven years later!
For those headshakers, dont knock if you haven't used it!
There is an easy way and a long way to most things with Alfa's
Simple, although many will over complicate the issue, a little bit of lateral thinking goes a long way instead of following what someone else does and thinking thats the only way to cure an issue!