It's from the actual calipers and discs! The lever uses a hydraulic light switch rather than the click type.
If you look at the discs you can see them moving ever-so-slightly when it clicks...this is when stationary btw they don't click when pushing the bike nor is there any juddering through the lever when braking...
I think the clicking might be because there is still air in the back of some of the pistons. So the pistons are not coming out evenly to meet where the disc is and are instead moving the disc to where the pistons actually are.
The pistons should always come out to where the disc is, so if the disc is not central between them, then the pistons on one side will come out further than the pistons on the other side, albeit only slightly.
Could of course also be the gremlin of the pads catching on the caliper, that we found before.
Defo needs further investigation, preferably when I am not hung over to fook like I was on Saturday