For awhile now I have been hearing a clicking noise that started very faint and over time it got louder and more frequent. It happened going forward or pushing it backwards. I have changed my brakes out for dual piston version and was thought maybe that somehow started this with the combination of a bent disc. That was ruled out, then maybe it was because of all the traveling I do any maybe bent something. During this last event the clicking was so loud I thought the wheel was going to fall off. At this point all fun stopped and wasnt going to move until things were figured out. Turns out both inner and outer bearings were bad, inside super bad, outer just bad. Google and amazon to the rescue, ordered 10 replacements, cheap like 15 bucks. Replaced the 2 and the clicking noise is gone! I have an early Nano and I am not sure if I have the same rims as everyone else so that is the reason I didnt provide more bearing details. The bearing has or should have some numbers stamped on it, google that and u should find them easy enough. I believe mine were 16100 2RS just so you have an idea what to look for.
So, if you hear a clicking noise around the mains, this is a good place to start.