My last "clatter in some gears but not others" was when I broke the Dura Ace cluster (some of the gears were triples, riveted, and the thing broke at the rivets) climbing a hill*. Something to check, anyway. Good luck.
*Not bragging, as in "look at me, I broke a Dura Ace cluster!" More like "Gee, I weigh a lot more than I did when I was 20", and "Gee, Shimano kind of over-engineered that cluster for weight savings." They've since redesigned it.