Bicycle frames transmit vibrations (creaks) very well, so where you think it's coming from might not even be close to the real source.

If you stand up and pedal, does the creak stop? make sure your thighs aren't rubbing on the saddle or this test won't help diagnose the problem. If it only creaks when you're seated, you might try some grease or assembly compound on the seat rails. If that doesn't help, use assembly compound on the seat post (no grease).

If it creaks seated or standing, does it creak while pedaling or while rolling? I'll guess it doesn't creak while sitting still. If it creaks while not pedaling, try tightening the skewer. If that doesn't do it, try a drop of oil on the NDS rear spokes where they cross. if it creaks only while pedaling, is it a cyclical noise - like every rotation of the crank? If so, try pedaling with one leg at a time and see if you can isolate to one side or the other. Loose cranks could be it, but it could be a pedal as well.

Ride with no hands on the handlebars - does it still creak? if the creak stops, the issue is the handlebar/stem joint, or the stem/steerer joint. Assembly compound will be helpful in either case. if this is the area, you should be able to make it creak by twisting the handlebars while stopped or when moving.

Maybe this will help.