I've encountered the sign before: a drivetrain click sounding
arrhythmically to the pedalling cadence can be an indication that a
chain pin is backing out. Every time the afflicted link rounded the
derailleur and cassette it clinked in protest.
It was dark with the mercury hovering at -20C, but there was slightly
over a kilometer to go in the commute. I thought I could defer dealing
with the problem; in a few minutes warmth and light would assist in a
routine fix.
I thought wrong. Grinding up the last hill of the ride, a highway
overpass, the pin popped out. The liberation occurred on the top run of
the chain (under load), on a pin fastening the leading edge of a pair
of outer plates. With me still pedalling, the front half of the link's
outboard outer plate swung free from the inner plate and hooked the
front derailleur cage as it approached the chainring. (The pin was
still inserted in the inboard side of the link.)
The cranks seized to the jarring accompaniment of mutilating metal -- I
hate those sounds. S&*t!
Ridding the chain of the malignant link took minutes, but the old
Tiagra derailleur was fubar. I wedged various appendages of my
multitool into its cage to pry open a route for the chain and confined
myself to one chainring for the rest of the day. The Tiagra was
subsequently retired in favour of an old Suntour gathering dust in the
spare parts bin. The bike's front (barcon) shifter is friction: no
shifter/derailleur compatibility issues.
In Toronto this has been a brutal winter for cycling.