I have City 5 Plus but this would probably apply to all hydraulic rear brakes. Have had bike 1 1/2 years with no problems. Rear brake just started making a clicking sound when applying hard full brake. Sounds like it's coming from disc/drum area?? When lightly applying brake, there is no clicking. I tried to duplicate the sound while stopped by leaning bike up on kickstand, using full throttle and then applying brake. No clicking. I can only hear it when rolling at a good speed and applying brake. It clicks until come to complete stop. Maybe something rubbing, need adjustment, ??
A clicking sound on my MiniST2, and others I've seen in videos, may very well be loose spoke(s) - try pulling on each one and see if you can feel a difference. If you find loose ones, there are videos on YouTube showing how to properly tighten them back up - be sure you have a properly-fitting spoke wrench tool.
I second the loose spoke idea. Just squeeze each spoke together with the adjacent spoke until you find the loose one. Tighten it up.
I did it on my bike at 300 miles.
Also, even if something else is causing it, it is always a good idea to check and tighten the spokes. The rear wheel gets more stress with the motor so the spokes there gets loosen first.
I had an issue with a dirty chain. It would click. Once cleaned no more clicking.
inoxa, what's a dirty change? Do you mean a dirty chain?
yes my error....chain