Are you certain it is Crank/BB area?
Saddle creaks can appear in various places on the bike- as can Bar stem creaks. So can broken tubes and damaged spokes on the wheels. All of these "Have" been the cause of mysterious Crank noises so check them out aswell.
But if you can't find the noise in 5 minutes of searching- get the bike back to the LBS. Let them spend their time in Finding it?
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How long was I in the army? Five foot seven.
Spike Milligan