As a chain wears down, it will start flexing more to the sides which may cause it to start rubbing the cog next to it.

Solutions:
- Measure the chain for stretch with chain stretch tool or straight rule. If stretched beyond acceptable parameters, replace chain.
- Avoid "cross-chaining". Avoid shifting to the outermost cog of the cassette while the chain is on the smaller, inner chainring. If you have 1x drivetrain, move your chainring outside of the crank arms so it becomes more aligned to the outermost cassette cogs.
- Get better quality chain next time with less sideways flex. Lower quality chains tends to flex more to the sides and may result to slow or delayed shifting between cogs. More flex will also make chain more likely to rub against adjacent cogs.