Some time ago I purchased some Nitto Bosco bars which are 22.2 mm diameter with a 25.4mm sleeve at the mounting point.
They creaked annoyingly and sat in my parts bin for a few years.
I had forgotten about the creak and recently fitted them to another bike.
They are just the right shape but the creaking was a real pain.
Corey at Rivendell had the solution!
“dripping a little loctite along the edge of the sleeve on each side and using some compressed air to blast it in there usually cures it.”
I was a bit skeptical when the loctite seemed to go everywhere but inside the sleeve.
However it worked!