Yes, I mean the 11 sprocket (smallest).
I can't replicate the problem on my repair stand. It's only happening while riding.
I have removed the cassette and re-installed it. I still have the same "rubbing" noise.
I didn't bent the derailleur hanger or damaged the cabling. It's shifting perfectly.
This is getting really strange and annoying
Do you mean the big chainring and the smallest sprocket? Smallest sprocket will be an 11 or 12, and 25 your biggest.
1/ The aluminium hanger that the rear mech connects to is designed to bend if you crash or if your bike topples over. These often bend inwards towards the back wheel, and your shifting stops working properly.
2/ Or maybe your freehub has come loose and when the chain is on the smallest sprocket the chain is rubbing against the aluminium derailleur hanger. So, if you remove the wheel and pull on your set of sprockets a little, you might see what I mean. You only need pull a little. In order for the freehub to be tightened onto the hub you will have to remove the sprockets.
3/ Or your shifting is just out. Have you damaged the cabling to the rear mech?