The only time I had random racket on some cogs and not others, my lock ring was not fully torqued. But you said you already checked that.
If you put it on one of the noisy cogs and then look from behind the bike, how does the derailleur alignment look? Is it off to either side slightly or directly under the engaged cog? If off, which side is it off towards? If you then put it on a different noisy cog, is it off the same direction?