Nevermind! Found the creak that was bugging me - the non-drive-side bottom bracket cup was no more than finger tight. The square taper crank bolts were also suspiciously easy to get off. And finally, there was no grease (and a bit of rust) between the seatpost clamp and the frame!

Only spooky thing is that, after cleaning the seatpost clamp, very lightly greasing the outside of the frame seatpost tube, sliding the clamp on (so it rests on the little fully extended set screw?), and tightening it securely down... the seat post won't slide through, it hits the rim of the seat tube!

I loosened off the clamp, and the seatpost slides in fine. What seems to happen is that as soon as I tighten the seatpost allen clamp bolt more than 5-10 ft-lb, the seatpost starts to get a bit difficult to slide... Anyone know if this is a problem, or should I just leave it loose-ish?