I’ve had that exact noise, actually swapped frames to get away from it - used the same wheels though so it came back!
In my case it says an alloy freehub that was allowing enough movement on the easier gears to allow them to rub. Swapped the Freehub for a steel one and the issue has gone. If that’s an option on those wheels, I’d definitely recommend it!
I’m here one year later, but still someone might find this usefull, as I’d be damned if there weren’t dozens of ppl having this problem.
Mine started creaking after around 1000 km. I pretty much tried everything as the OP with no avail.
Then I found the service manual: https://www.sram.com/globalassets/document-hierarchy/service-manuals/zipp/wheels-and-hubs/zipp-cognition-v2-hubs-service-manual.pdf
..where on the page 4 it mentions this:
And as I had nothing to loose, I gave it a shot andd performed the “Every 100 hours” service.
The only kinda problem was obtaining the Zipp Cognition Oil as on the most of the websites it was out of stock or on backorder, etc.
Eventually I got one and performed the hub clutch removal/installation.
It’s very easy procedure and most of the users can do this at home, trust me.
When I took it apart I was surprised by how little or almost no oil was in the clutch compartment.
During installation, the service manual says "Use a small syringe to apply approximately 1 mL of Zipp Cognition oil … " …so here I’d like to put emphasis on the 1 mL which IMHO looked not enough. So I gave it like 1,5 almost 2 mL. And yes trust me, it’s fine, there is enough of room to take it
Anyway, so I assembled it together and here comes the celebration part, the creaking was gone 
BTW #1: Perhaps any oil would work, but I didn’t want to risk it especially since I was able to obtain the original Zipp oil fairly easily.
BTW #2: Still I’m unsure which part of hub clutch was making the creaking noise, but I’m roughly betting on that pink foam springy ring not being oiled enough and rubbing over the clutch. But that’s just my guess..
Hey @Bedas thanks a lot, this totally resolved the noise problem. There was no oil at all, as suggested I have added 2ml and now no more creaking noise!!
I have ridden with this noise for more than a year!!
Bad news…
After approximately 1000 km, the issue started again. At first, it started creaking only sometimes now and then and just went away. It was very random. But ultimately, it was the non-stop creaking orchestra all over again.
I looked at the center part of the hub shell and it was dirty of the oil. So the oil leaked thru the Clutch Seal Cap out. But very strangely as it was focused on one side mainly as if it all leaked in one go. While one would say, that if it’s been leaking slowly continuously it should be spread all over the hub sheel body.
Either way, I did the same maintenance as above and it’s all good again …. but tbh, I’m not very pleased with those wheel…well, mainly with this flipping freehub.
