Turns out it was the seat tube sleeve. I always keep my seatpost clean and lightly greased, so I thought I had that area covered, but I forgot about the sleeve. I noticed because my stand is a bit of a cob job involving using a piece of seatpost that goes into the seat tube. After thoroughly cleaning and greasing the appropriate parts on the headset for the second or third time, I noticed that when I rocked the frame around in the stand, it would make the same creak. Cleaned out all the sand and applied some grease and the creaking is all gone. Enjoyed a quiet ride for a change.