Headset help

Its driving me mad!!! Rebuilt my road bike in the last month or 2 and all was smooth until the headset started creaking like Carl Beck's knees!!
Ive had it in/out shaken it all about, more grease, less grease and no matter what it wont stop creaking. It only happens when out of the saddle and cranking it hard. It was a new headset that went in, new crown race, new star fangled nut and all looks good as far as I can tell, any clues anyone?
Before someone mentions it, it is definitely the headset, I initially thought it was the BB but is the headset.

James, is your frame a Planet X?

I have the SL Pro and I also have the head tube creak!

I've had the bung in and out a few times, used carbon assembly paste, torqued all to spec and even though the creak is less it is still apparent when out of the saddle.

I wonder if it is flex in the frame?

My wheels on this bike are Mavic Ksyrium and Mark suggests extra grease under the dust cover and on top of the frictional bearing on the front wheel but as i've spent the morning replacing the crank bearings on an ultratorque - wavy washers (I ask you), fingertip shredding C clip and seized Hoth joint I've no appetite to get into the Mavics this afternoon.

Cheers for the reply Martin, but alas, its not mt TT bike, its my old 2009 Felt Z85. Its a carbon fork but has an aly steerer with a star fangled nut in it. It was fine when I first fitted the new headset and only started playing up after a few hundred miles or so.
Its driving me mad!!!