Worryingly I have found this on the internet, exactly my problem!
"The bike started making a creaking noise that I thought was the BB. Noise was only under load, but didn't require major force, ie, riding slowly up my shallow driveway in first or second gear would do it. I was able to duplicate the sound in the stand by lightly braking the rear wheel. The sound was sort of like a derailleur clicking against a spoke. It occurred 2-3 times per revolution, but this was variable. Replacing the wheel with another made the sound disappear, only to return the minute I put the wheel back on.
Strangely, the noise goes away after the bike has sat a few days, but then when I am several miles into a ride it picks up again and gets louder as the ride goes on."
"i had two sets of easton vistas that did the exact same thing. my trusted lbs mechanic could never quiet them and neither could i. it would stop after the bike set for awhile but like you said start up again and get louder and louder. my mechanic said it was the freehub internals were just flimsy and delicate. we tried oil, grease, no oil, no grease, new bearings, etc. etc.
i finally ditched both and won't be bothering with any more easton wheels...."
I think I'll have to ditch them too. bah!