Bike noise, creak, click, creaks, clicks, clicking, pinging sound - help

Ok guys. I am having the MOTHER of all creaking nightmares.

I'll start by telling you the things I have done so far:

-replaced BB (external type) with fresh grease and a check over by a bike shop
-replaced and regreased chainring bolts
-replaced and greased seatpost clamp (and sanded the area beneath the clamp to get rid of paint chips)
-regreased seatpost
-replaced (twice) saddle, regreased rails and the mounting points of the rails to the saddle body
-rebuilt front end, put grease everywhere that was sensible - on all bolts, even tiny bit between the stem and bars
-replaced pedals, regreased pedal threads.

Bike is an '07 condor lavoro (aluminium) with full carbon forks.

Still creaks in exactly the same way, repetitive, rhythmic clicks while pedalling (fixed). Seems to happen at a particular point in the rotation of the pedals, and I can make it happen by sitting on the saddle, feet on pedals, rocking back and forth.

It DOESN'T happen when i'm out of the saddle, ever, but it does happen when I'm riding no handed. Everything tells me that it has to be something to do with the saddle/seatpost/clamp... but I'm at a loss.

Oh yeah, and it's impossible to actually tell where the click is coming from. It's travelling through the tubes of the bike, sometimes it sounds like it comes from the saddle, others from the front end...

Only things I haven't done yet are replace the chain/rear sprocket, or have a serious look at the wheels, primarily because of the above.

I can't find any cracks or deformations, just the odd paint chip here and there. I'm really getting worried there is something wrong in the seat tube that I haven't found yet.

It is literally driving me mad. Any help much appreciated.