Recently I noticed my chain is rubbing against the cage on one side of my bottom pulley (9 speed Shimano 105 RD).
It's not forceful, but definitely brushing it when I'm in a middle chainring - large cog combo; the chain is more centered in a middle-small combo.
Only thing I noticed is that pulley wheel appears to have a lot of side to side play relative to the other pulley or the top pulley on my other bike (on that one I can push it side to side and can't get the chain to touch the cage). I stuck an allen key into the pulley hole and it appeared perfectly tight, not sure what else I can do - I've never taken a pulley off.
Is it possible I'm barking up the wrong tree (rear derailleur alignment off?) or is it normal for these pulleys to wear out? This is original equipment, 5 years old probably ~40k miles.