My Strada just started doing it recently, and only when I am rolling the bike backwards down my path and I hit the front brake.
If stationary and I push forward up and down on the forks with the brake on, I hear nothing. As soon as I pull or roll the bike backwards with the brake on there is a single click like knock.
I have experienced this with bikes in the past and it has always been the headstock bearings, slightly loose for some reason. Previously a quick tighten up on the bearing has cured the problem but I haven't tried it on my Strada just yet.
I intend to investigate further and get my apprentice (GF) to move the bike while I listen more closely with a stethoscope, but as the noise was not there from new and has only developed 4000 km into shoddy roads I am pretty much convinced my noise is head bearings and can be easily fixed - I hope