This has been bugging me for a while. On my 11-28 cassette only the 12t makes a lot of noise. Sounds like it's not adjusted right but no amount of adjustment has an effect and the overall shifting is fine where it is. On closer inspection it looks one of the teeth on the 11 is slightly closer to the 12 and the chain catches momentarily. I don't know if that is the reason for the noise but it's a good candidate. I can't imagine how you would bend a single tooth like that and the bike has not been crashed or even bumped.
Has anyone seen this before? Is it possible to fix? I haven't tried to buy just one cog but could probably pull one off an older cassette.