Quote Originally Posted by Becky View Post
The chain jumps and rattles, but only in the 12-tooth cog. The rest of the cogs are nice and quiet. Everything shifts fine, so I'm thinking it's not a cable tension issue.
Even if it only jumps on the small cog and shifts fine it well could be a cable tension issue. That small cog seems more susceptible than others to any slack in cables. (I just changed my cables and was having the same issue after the second ride - only in the small cog - and the bike shifted just fine. Things had stretched and as soon as we pulled the cable tighter it worked great.)

If you have a new bike build - and new cables - there may have been some stretching.