This has been posted in the 145/146 section so hopefully a moderator will move it to the correct section for you.
I'm not familiar with the 156 other than the suspension wishbone setup on the front is completely different. What might be similar though are the suspension strut top mounts. If they're anything like the 145/146 then the bearing under the top mount eventually disintegrates or the bonded rubbber insert in the top mount breaks away from the metal body of the mount itself. The result is knocking when going over bumps and/or when turning the steering wheel left to right (which sounds exactly like the issue you're describing). The fix is to replace the top mounts.
I won't be posting on this forum any more. If you want to know why and/or you want answers to some 145/146 related questions then please feel free to pop over to and post on the alfa145.com
forum where you will find me with the same member name.
I have now disabled all my private messaging options on here but you can PM me over on the alfa145.com
forum if you need to.