Originally Posted by
Lascauxcaveman
^+1
I've set up RDs with no slack at all in the small cog position and had the same problem. Backed off the RD cable a bit so there's no tension in that last position and the problem went away.
Yes, worth looking at, if that is not it...
Take a gloved hand (I really don't like getting my hands filthy on purpose) and twist the chain to see how much it flexes in rotation axialy. Always a chance the chain is toast. I recently had that problem on a bike I bought, was not stretched, but was way too limber the way I described. New chain changed all.