Cassette jumping/rattling only in smallest cog?

[lunacycles]

This sounds to me like your cassette lockring has come loose, so the smallest cog (at least) is rattling and not staying totally stationary. Would also explain the chain jumping around in that area.

Another possibility is that there is a spacer missing near the 12t cog, or the wrong spacer installed.

[SadieKate]

Dang, Margo beat me to it.

[aicabsolut]

This sounds to me like your cassette lockring has come loose, so the smallest cog (at least) is rattling and not staying totally stationary. Would also explain the chain jumping around in that area.

Another possibility is that there is a spacer missing near the 12t cog, or the wrong spacer installed.

Has it always done this? I was thinking something funny with the cassette or freehub too...

But...if you've been making a lot of adjustments for "cable stretch" (which is really a myth), then you may have overtightened the cable, such that it wants to jump to the 13 cog when you're in the 12.

Since I can't see if it's trying to shift or whether it's just rattling, then my first guess would instead be that the H limit is not adjusted properly. Rattling on the extreme gears only typically indicates a limit screw problem.

Note: to adjust the limits, you should undo a lot of that indexing (i.e., loosen the cable). Then redo the cable tension after the limits are set. To test the H limit, pull the cable under the downtube to shift to the 13 cog, then let go to let the chain shift back to the 12, and check for how the chain rides on that cog again. So, by starting over this way, you'll fix the problem if it's a cable tension or limit screw problem. None of that will do a bit of good if it's due to improper cassette installation, obviously.

[DebW]

Could be you've overtightened the cable so it can't reach the 12 properly, and isn't even hitting the limit screw. Back off the cable tension and see what happens.

[Becky]

You guys are gonna love this.

I am a moron, and was turning the low limit screw when I should have turned the high. Now that my head is firmly screwed back on, all works as it should

FWIW, I did find that the lockring was a little loose too. So maybe it was a combination of things. That's what I'm going to believe, because it makes me feel slightly less foolish