The only thing that creaks when out of the saddle on my fixie Swift are my aged knees.
Seriously, though, the linkage in the rear traingle of the Friday is most likely the culprit. A curious Friday owner stopped me on the train one day to examine my Swift. He said the rear triangle on the Swift is much stiffer than that of his Friday. I wouldn't worry about it, though. It's a great bike, and the legions of faithful Friday owners out there can attest that the bike is reliable and safe.