Yes, that ought to be a good test; put the front axle in line with the frame's long axis! Thanks!

When I was messing about with this bike yesterday, after mounting a longer stem than the one it came with, I eventually could see a little bit of motion of the front pads when they were held in contact with the front disc. The motion was timed to accompany the slight clicking I'd been hearing, that I thought could be from the headset not being properly tight.

No movement was felt at my fingertips when I had them bridging the seam between the fork bridge and the bottom bearing housing end of the headset tube.

I'm pretty certain now the noise was the pads shifting slightly in the caliper, moving with the disc while being restrained by their cotter pins.