ps, just read the responses after I saw Nuns post, as I hadnt read them. ya its probably derailleur alignment a bit, but I should clarify that I "feel" a difference through my feet slightly (and no, its not as blatant as the chain hitting a screw or whatever) and Im pretty sure I am feeling the chain being more "bunched" around a smaller diameter--but I admit I am talking about a small difference here.
Sorry if I suggested something that may seem trivial like hitting a screw. Now offense was meant by it.
Perhaps you are more sensitive to chain noise/vibrations but I'll offer some other suggestions. We don't ride in the 11 tooth cog all that often so it doesn't wear as much as the other cogs which could make it mesh differently with the chain. There's a bit of wobble in the freehub of all bikes which could be exacerbated by cog being the furthest outboard on the freehub. A slight misalignment or a bit of cable play could make the chain not mesh quite right as I stated above. A bit of twist to the derailer hanger could also mismatch the cogs. Or it could be any combination of things.
Perhaps it would be best to do the thing that you do with a car that has a minor but nagging problem...turn up the radio
