Mix up a small batch of oil and light petroleum distillate -- OMS, naphtha, lighter fluid, diesel fuel, or even gasoline if using outdoors. You want a high solvent ratio so it pours like gin. Use a dropper to apply between post and frame and let it wick in. Ride the bike. If the creaking changes or stops, you know the issue. This quick job ill last a while, and repeated applications will eventually result in a permanent solution. If you want, try using a heavier oil, with more thinner, which would leave more residual film.

BTW- for my own edification only, am I right in assuming that this is a frame where the seat tube extends up above the top tube by more than an inch or so?