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Years ago I had a 99/00 Trek hardtail that quickly developed an awful seat tube creak. It turned out to be a known issue with that year's run – the seat tube had a shim in, welded at the top, so you could run a 27.2mm post in the wide aluminium tube. This would pop loose slightly somewhere, rubbing, to make an unholy racket that sounded like the back of your bike was about to drop off. The 'cure' was to turn the bike upside down, spray WD40 in through a seat tube bottle boss, let it run down the inside of the frame and lube the creaky area. The problem would go away (temporarily). I ended up getting a warranty frame out of them for that and other reasons. Trek subsequently omitted the shim and moved to 31.6mm seatposts.
I've had my current Cannondale CAAD8 road bike for four years now. When, after a year, it started exhibiting the same creaking symptoms as the old Trek, I tried the same solution – and it worked! Not a seatpost/clamp/saddle issue, definitely a problem within the seat tube – perhaps even the same problem. Sometimes the 'cure' lasts for weeks, sometimes only a couple of rides. No external signs of cracks round the top tube/seat tube/seat stay welds., etc.
I'm thinking of a more permanent solution the next time I strip the CAAD8 down. Back with the Trek, there was talk at the time of pouring loctite in through the bottle boss rather than WD40. I could see that working. Any thoughts of what specific bonding compounds I could use to do this, that wouldn't end up with me having an unusable frame if it goes a bit wrong?
Not sure about loctite.
Did hear linseed oil was good though. And smells of putty and cricket bats.
Before you go the whole hog and try re-bonding you could try ACF-50. It's like a thinner waxoyl with a very good ability to "creep" into crevices and stay put.
Ok, might try that first. It's an easy enough maintenance job, I'd just prefer to be doing it every couple of months rather than every couple of weeks.