Noises that seemed to suggest some problem with my bike, but turned out to be some laughable trifle:

a. The left crankarm's being in contact with the end of the front derailleur cable.

b. Dirt/chunck of mud/leaf stuck on the tire.

c. A sticker on the chainring of a brand new bike that was not peeled off completely.

d. The load on the rear rack touching the tire.

e. Helmet rubbing my back collar (because my ears were covered by the helmet, it soundes like a remote noise).

f. A jogger nearby, whose steps somehow followed my pedaling cycle.

g. Birds

h. Yet to be figured out--clicks when rear derailleur was on "3". I was never able to pinpoint where the noise came from.

Can you add some other causes that turned out trivial?

Edit: Add:

- Reflective tape that I put on the crank started peeling off and touching the chainring

- Bottle cage under the downtube too close to the crank when pedaling, touching it when the cage is not empty.

- Leg hitting bottle cage