Originally Posted by
FBinNY
OK, it's a squeak from something moving. Since it happens even when not pedaling we can rule out things like the crank or drivetrain, and even the rear wheel shifting under load.
So, try to find anything that can flex or move sightly under your weight. Could be within the saddle, the saddle in the clamp, the post in the frame, a loose headset, the handlebars or stem, and so on.
Try standing beside the bike with the front brake on, and pushing back and forth, flexing the saddle sideways, and every possible way to "massage" every part until you can reproduce the squeak. Once you can reproduce the creak, you can localize it to it's source.
Warning, this can be a long slow process, since mechanical objects can be like people in that they don't like to do wrong while under scrutiny. Once, I had a similar issue, where I couldn't localize it. I ended up taping a small memo recorder to various places on the bike and riding, until I found that place where it was loudest, and was able to work from there.