I had exactly that creak and it drove me nuts - was convinced it was the headset/front end at first, did exactly what you did to find it - funny thing is when it first started on a ride i stopped by my LBS on the way back as i thought i was going mad and wanted a second opinion.
The mechanic (who is a proper mechanical anarchist and top notch wrencher) leaned the bike over stood on the down tube and started flexing the hell out of the frame to see if it was any of the linkage bolts - he was actually pretty impressed with the flex in the rear end it as he thinks a lot of bikes can be too stiff and a lack of flex in the frame means linkages getting far more abuse, and that overly stiff bikes don't track as well.
Was pretty eye opening
The mechanic (who is a proper mechanical anarchist and top notch wrencher) leaned the bike over stood on the down tube and started flexing the hell out of the frame to see if it was any of the linkage bolts - he was actually pretty impressed with the flex in the rear end it as he thinks a lot of bikes can be too stiff and a lack of flex in the frame means linkages getting far more abuse, and that overly stiff bikes don't track as well.
Was pretty eye opening