If you hear a click/creak every 2-3 pedal strokes chances are it is a tight/un-lubed link in the chain. Look at the chain and pedal backwards to identify sticky links.
If you unweight your pedals, and still hear the creak when you hit bumps, chances are it is your saddle rails or seat post. Grease the seat post and seat tube opening generously. If you have a removable seat post clamp, take it off, apply grease to the outside of the seat tube and reinstall. Take your saddle off, apply a thin coat of grease to the rails and reinstall.
Another culprit to a creak when the pedals are unweighted is a crack in one of the frame welds. This can be a lot harder to find, and much more expensive to remedy.
If you hear the creak only while applying pressure to the pedals, and hear it every pedal stroke, it could be your cassette/freehub, bottom bracket, cranks or pedals. Take your cassette off, grease the freehub body and reinstall the cassette. Take your bottom bracket out, grease the hell out of your frame's bottom bracket shell, the bottom bracket cups, pretty much everything but where the cranks slide onto the spindle. Make sure all your chainring bolts are tight. Remove your pedals, grease the threads and reinstall. If you are running toe clip type pedals, ensure the bolts holding your pedal cage on are tight. If you are using clipless, clean the cleat trapping mechanism well (this is the one part of the bike you can use a high-pressure water hose to clean) apply wax/oil to anything and everything that moves.
Cheers,
Juba
