Might also be a sign of a slightly bent derailleur hanger.

Another thought - chain can make some noise on less used cogs; the frequently used cogs would be worn down a bit together with the chain, while the less used are still like new and don't mesh perfectly with a slightly worn chain anymore. That kind of wear can be invisible to the eye. However, it's usually the smallest and largest cogs that see less use than the middle of the cassette, but it might vary with gearing, preferred cadence, terrain etc.