I ride an SE Lager fixed. On the 30th of May this year I did a bit of an overhaul, replacing the rear cog, chain, wheels, tyres and brake pads.
I measured the chain last weekend and discovered that it was already over .75 (which I asked about here), but less than 1.0. So I decided the replace the chain with a new KMC Z410 chain. I use a Surly track cog 1/8 inch, 17t on the back and since it was only a few months old and with a single chain I decided not to replace it. The chainring is also not old and looks to be in fine condition.
Here is a photo showing the old cog (on the left) and a brand new cog (on the right). I couldn't see any appreciable difference which I took as reinforcing my decision that it didn't need replacing.
Unfortunately the next day I went for a ride the drivetrain was incredibly noisy. Clunking and grinding. I could even feel roughness through the pedals. I was thinking maybe the bottom bracket (the only drivetrain component that has not been replaced yet) might have completely fallen apart. What is causing this noise?
Update: I should clarify that I don't have the answer to the question. I know how I fixed it, but that doesn't preclude another answer being more informative