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Its a Thomson post in carbon frame. Think the post is slightly smaller than the internal diameter of the seat tube. Creak normally happen when standing up off saddle. Post is well 'greased' with carbon paste. Seat collar well lubed, a correctly torqued.
Solutions seem to be
1. Get another brand of post with hopefully a slightly larger diameter
2. Apply some form coating to the post (possible a possibly a clear coat laquer) to built up the diameter slightly.
3. Put up with it!
I had similar with a commuter bike, I am pretty sure it was the seat tube being oversized rather than the post too small but no amount of carbon paste would stop it creaking and slipping slightly. In the end I made a "shim" from a 2L lemonade bottle and it solved the problem.
The lemonade bottle may be too thick but you could try something thinner.
Could try a coke can shim, but I suspect the tolerances are so tight that I will never get the shim in (or out when the time comes).
I doubt it's the post .. Thomson are pretty finnicky
I guess instead of lacquer you could try and put a thin coat of araldite and let it dry then rub down until it goes in but is good and tight?
If its a carbon frame worth checking to make sure its not got any spaces (or even if its round)..though measuring internally would be a real pain.
Does it creak monetarily as you unweight the post, or whilst standing?
Hitting a bottle cage mount?
This was the source of my hard to find creak.
Shorter post sorted it.
Coke can shim doesn't have to go right round the post - put a strip down the back like a shoe-horn, slip the post in. Gently score round the excess sticking out with a knife and rip it off with pliers.